This issue has been added to the tracker:
https://winscp.net/tracker/1646

WinSCP cannot currently provide enough information to the extension/custom command, so that it can bypass an invalid certificate.

You would have to make a copy of the extension script and modify it to ignore invalid certificates.

Hi there,
I want to use the static customized command: keep local directory up to date. Having clicked it and the script returns below error message:
. 2018-04-26 20:45:59.255 --------------------------------------------------------------------------
. 2018-04-26 20:45:59.256 WinSCP Version 5.13.1 (Build 8265) (OS 10.0.14393 - Windows Server 2016 Datacenter)
. 2018-04-26 20:45:59.256 Configuration: nul
. 2018-04-26 20:45:59.256 Log level: Normal
. 2018-04-26 20:45:59.256 Local account: EC2AMAZ-9H3GNFK\Administrator
. 2018-04-26 20:45:59.256 Working directory: C:\Program Files (x86)\WinSCP
. 2018-04-26 20:45:59.257 Process ID: 1216
. 2018-04-26 20:45:59.257 Command-line: "C:\Program Files (x86)\WinSCP\winscp.exe" /xmllog="C:\Users\Administrator\AppData\Local\Temp\2\wscp1338.03A88F34.tmp" /xmlgroups /xmllogrequired /nointeractiveinput /dotnet=5.13.1 /ini=nul /log="C:\Users\Administrator\Desktop\KeepLocalUpToDate.log" /console /consoleinstance=_4920_12219842_363
. 2018-04-26 20:45:59.258 Time zone: Current: GMT+8 (China Standard Time), No DST
. 2018-04-26 20:45:59.259 Login time: Thursday, April 26, 2018 8:45:59 PM
. 2018-04-26 20:45:59.259 --------------------------------------------------------------------------
. 2018-04-26 20:45:59.259 Script: Retrospectively logging previous script records:
> 2018-04-26 20:45:59.259 Script: option batch on
< 2018-04-26 20:45:59.259 Script: batch on
< 2018-04-26 20:45:59.259 Script: reconnecttime 120
> 2018-04-26 20:45:59.259 Script: option confirm off
< 2018-04-26 20:45:59.259 Script: confirm off
> 2018-04-26 20:45:59.259 Script: option reconnecttime 120
< 2018-04-26 20:45:59.259 Script: reconnecttime 120
> 2018-04-26 20:45:59.259 Script: open ftp://xxxxx:***@xxxxxx.componize.com:2121 -explicit -passive=1 -timeout=15
. 2018-04-26 20:45:59.259 --------------------------------------------------------------------------
. 2018-04-26 20:45:59.259 Session name: xxxxxx@xxxxxx.componize.com (Ad-Hoc site)
. 2018-04-26 20:45:59.259 Host name: xxxxxx.componize.com (Port: 2121)
. 2018-04-26 20:45:59.259 User name: xxxxxx (Password: Yes, Key file: No, Passphrase: No)
. 2018-04-26 20:45:59.259 Transfer Protocol: FTP
. 2018-04-26 20:45:59.259 Ping type: Dummy, Ping interval: 30 sec; Timeout: 15 sec
. 2018-04-26 20:45:59.259 Disable Nagle: No
. 2018-04-26 20:45:59.259 Proxy: None
. 2018-04-26 20:45:59.259 Send buffer: 262144
. 2018-04-26 20:45:59.259 UTF: Auto
. 2018-04-26 20:45:59.259 FTPS: Explicit TLS/SSL [Client certificate: No]
. 2018-04-26 20:45:59.259 FTP: Passive: Yes [Force IP: Auto]; MLSD: Auto [List all: Auto]; HOST: Auto
. 2018-04-26 20:45:59.259 Session reuse: Yes
. 2018-04-26 20:45:59.259 TLS/SSL versions: TLSv1.0-TLSv1.2
. 2018-04-26 20:45:59.259 Local directory: default, Remote directory: home, Update: Yes, Cache: Yes
. 2018-04-26 20:45:59.259 Cache directory changes: Yes, Permanent: Yes
. 2018-04-26 20:45:59.259 Recycle bin: Delete to: No, Overwritten to: No, Bin path:
. 2018-04-26 20:45:59.259 Timezone offset: 0h 0m
. 2018-04-26 20:45:59.259 --------------------------------------------------------------------------
. 2018-04-26 20:45:59.261 Connecting to xxxxxx.componize.com:2121 ...
. 2018-04-26 20:45:59.561 Connected with xxxxxx.componize.com:2121, negotiating TLS connection...
< 2018-04-26 20:45:59.649 220 FTP server ready
> 2018-04-26 20:45:59.649 AUTH TLS
< 2018-04-26 20:45:59.740 234 Switching to TLS secure session
. 2018-04-26 20:45:59.950 Server asks for authentication with a client certificate.
. 2018-04-26 20:46:00.040 Verifying certificate for "Alfresco Software Ltd." with fingerprint c7:50:c4:95:03:90:f7:5e:45:58:58:89:08:5f:d7:4f:1b:8c:c2:32 and 19 failures
. 2018-04-26 20:46:00.041 Asking user:
. 2018-04-26 20:46:00.041 **The server's certificate is not known. You have no guarantee that the server is the computer you think it is.**
. 2018-04-26 20:46:00.041
. 2018-04-26 20:46:00.041 Server's certificate details follow:
. 2018-04-26 20:46:00.041
. 2018-04-26 20:46:00.041 Issuer:
. 2018-04-26 20:46:00.041 - Organization: Alfresco Software Ltd., Alfresco CA
. 2018-04-26 20:46:00.041 - Location: GB, UK, Maidenhead
. 2018-04-26 20:46:00.041
. 2018-04-26 20:46:00.041 Subject:
. 2018-04-26 20:46:00.041 - Organization: Alfresco Software Ltd., Unknown, Alfresco Repository
. 2018-04-26 20:46:00.041 - Location: GB, UK, Maidenhead
. 2018-04-26 20:46:00.041
. 2018-04-26 20:46:00.041 Valid: 8/10/2012 4:21:00 PM - 7/17/2112 4:21:00 PM
. 2018-04-26 20:46:00.041
. 2018-04-26 20:46:00.041 Fingerprint (SHA-1): c7:50:c4:95:03:90:f7:5e:45:58:58:89:08:5f:d7:4f:1b:8c:c2:32
. 2018-04-26 20:46:00.041
. 2018-04-26 20:46:00.041 Summary: Self-signed certificate in certificate chain. The error occurred at a depth of 2 in the certificate chain.
. 2018-04-26 20:46:00.041
. 2018-04-26 20:46:00.041 Certificate was not issued for this server. You might be connecting to a server that is pretending to be "xxxxxx.componize.com".
. 2018-04-26 20:46:00.041
. 2018-04-26 20:46:00.041 If you trust this certificate, press Yes. To connect without storing certificate, press No. To abandon the connection press Cancel.
. 2018-04-26 20:46:00.041
. 2018-04-26 20:46:00.041 Continue connecting and store the certificate? ()
. 2018-04-26 20:46:10.044 Peer certificate rejected
. 2018-04-26 20:46:10.044 Disconnected from server
. 2018-04-26 20:46:10.044 Connection failed.

I saw some posts suggesting that adding certain switch may help, but the problem is I can find a way to modify the command.