It's indeed like you said Cazador. Serpentine99 told me this in a pm:
You have to add it like this:
first this is always the same:
http://www.thamnophis.com/caresheets/index.php?title=
Then you add the title, but with _ like you say. This is for the new page, how it will be called
Then you can add the title you want to be visible for the viewer, so without _ but with spaces.
And everything together you need to enclose it with brackets: [ ]
So for example:
[
http://www.thamnophis.com/caresheets...itle=Mouth_Rot Mouth Rot]