How to hookup MorphTargets for corrective animation??

I’m trying to create/hookup corrective morph targets to joints. As a test, I sculpted the unreal mannequin’s thighs to expand and contract.
I brought that into unreal, and now I see a slider which controls it in the editor. Great!

Now, I would like to activate the morph every time the character’s feet touches the ground. (Value to 1) When he lifts his foot, I’d like the flex to end. (Value to 0)

I’m thinking that I need to add a socket to the controlling joint, get it’s rotation values, and normalize it to drive the shape somehow… unless I can get the joint itself?

Any insight will be greatly appreciated