Hello! I was wondering how to use custom signpacks. Do you have to do the thing where you go and get the direct link for every sign or is there a better option?

Signpacks almost always include .loc files, so that the game knows to embed the link to each sign you choose. These are text files that include a URL with a direct link to the sign.

So you can just extract the files and place them in your Documents\Trackmania\Skins\Any\
(assuming the signpack creator has the "Advertisement" folders already set up within the pack)