I'm looking to have a feat that gives a class an additional known spell. so instead of Ranger knowing 3 spells at lvl 2 with this feat they get one more.... but i can't figure out how to do that, or if its possible.... i have been able to give hem a spell, but it is cast at will, but i dont want it to cast at will.

So, in the spells subsection, add the spell like normal, put number of uses to 1, using whatever modifier, reset at long rest. 

It won't add a spell slot to the sheet, but should have a Use button which will reset when you click long rest.