I am a user who uses this mod while playing Minecraft.
And I just installed it now, and I couldn't load my skin by URL.
What specific URL I have to put in the command?
For example, I tried to load a skin from http://minecraft.novaskin.me/gallery and https://www.minecraftskins.com/3/ but it is not working...what specific site can I put in the command?
Is it only working from using URLs from minecraft.net?

It should work from any skin site, just make sure you provide a valid url (usually ending with .png).

I guess you've figured it out already.