How to put the SSH Key in the Session URL?

There is a function in the menu to generate a nice URL that directly links into a SCP connection. The URL looks like this:

winscp-scp://ssh-user;x-name=MyNiceProject;x-publickeyfile=C%3A%255CUsers%255Cmyname...%255Cputtyformat.ppk@server.com/www/htdocs/

%255C translates to %5C which translates to "\". This means, the path is URL encrypted twice. Strange.

The URL opens with Winscp 6.5.3. Winscp thinks that the whole string "ssh-user;x-name=MyNiceProject;x-publickeyfile=C%3A%255CUsers%255Cmyname...%255Cputtyformat.ppk" is the username.

I converted the URL in a format that looks more like an actual URL:

winscp-scp://ssh-user@server.com/www/htdocs/?x-name=MyNiceProject&x-publickeyfile=C%3A%255CUsers%255Cmyname...%255Cputtyformat.ppk

Winscp can see the correct username now but completely ignores the key file. It asks for a SSH password.

I tried single encrypted path with %5C, I tried C:\\ and C:/ but nothing works. What is the correct format?

Plus: Is there any way not to specify the full path to the key file, as I don't know where exactly others store their key files?