\t represents a tab character. To use a literal backslash, use \\.
"C:\\Users\\pat\\Desktop\\WinSSHTerm\\tools\\plink.exe" -P 22 -l root 192.168.0.45 -nc XX.XX.XX.XX:22 -i "k ey\\key- ä4096.ppk" -pw 12345678
See https://winscp.net/eng/docs/ui_login_proxy#command

Hello,

I'm trying to set an absolute path to the plink executable. Somehow the string gets corrupted, so the connection cannot be established. The configured path is "C:\Users\pat\Desktop\WinSSHTerm\tools\\plink.exe". In the log you can see the corrupted path:

. 2019-02-17 18:10:23.561 Starting local proxy command: "C:\Users\pat\Desktop\WinSSHTerm ools\plink.exe" -P 22 ...

Thanks!