However, it requires "pkg-config" which is tricky to install in Windows and may not even work
-snip-
This is my error below both normal command prompt and in powershell run as admin.
'pkg-config' is required to install this package. Please see the README for details.
I tried to make this work since you seem to really need to use this in Windows but;
Even provided with a compiled "pkg-config.exe", nostrj will still error out during installation due to one of its requirement "secp256k1" having issues with Windows.
With that, I'll just suggest you to use Linux if you want to use joinstr plugin with Electrum. (or try other methods but I can't help you any further)