one way is to use pin (up left up middle - it doesn't matter that much for me). But since you want to lock it on a certain position on the bottom then be sure for which kind of bottom you mean. Screen bottom or website bottom so you can place it accordingly.
Another way is using scroll motion.
Set the key position to the top of your page (type or drag n drop the little blue line) Then place set initial motion to UP- 1px and then everything else (3 more entries) to 0px.
I hope that helps.