Scott the F is implicit Baxter#guitar players: how do you get better at fretting with your left thumb?
The Great Llama 
@eyesquash
Bigger hands? 😆
I have basically ALWAYS played the open D chord with my thumb wrapped around fretting an F# on the E string. You might try that. It's an easy way to work it into routine playing and get used to having it be a fretting finger.