Correct syntax with privatekey=

I have a private key file that is not working by command line. After typing below, I get asked for a password. There is no password, just username and key. What is wrong ?

open "sftp://username@10.10.10.10/ -privatekey=""D:\SFTP\Prod\ServerKey\private.ppk""

This is the correct syntax:

open "sftp://username@10.10.10.10/" -privatekey="D:\SFTP\Prod\ServerKey\private.ppk"

Though you actually need no quotes in this case, as there's no space in either argument.