Try removing the //.

See https://winscp.net/eng/docs/session_url#semicolons

Interesting. My projects are documented in a wiki. I've added deep links to the WinSCP connection to the respective project pages. I tried opening the URLs using the Chrome browser in Windows 11.

Where/how do you use the URL?
When I start WinSCP with the first URL from your post as the first (only) argument of WinSCP:

it correctly looks for the key in the C:\Users\myname...\puttyformat.ppk path:

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Regarding your question about path to keys: Where would WinSCP look for the keys? There's no standard key path.

For the archive. I couldn't fix this, the URL that is generated by WinSCP simply doesn't work.

But I set the SSH Key file as the default for new connections. Then an URL like this works:

This format has the advantage that it works even if other users have their ssh key files in different paths.

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

%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 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?