Go to your User CP
Click on EDIT SIGNATURE on the left of the screen
Then click on the box that is yellow & looks like it has a mountain & sun (it is tiny, BTW) & it is below the two arrows
Then another box will open & it will have HTTP in it but just copy your http & paste it in there over the HTTP
Click OK
Click preview signature so you can see if it showed up & then click save signature if you are happy how it saved.
I just tried it in my post & it didn't show up, it was a red X.