Lots of people take Tadalafil with Zoladex (or one of the other HT injections).
Only about 20% of men can get an erection on HT injections, and it's often short lived, and/or orgasm is elusive.
Tadalalfil certainly increases the number of men who can get erections on HT, but no one has researched by how much.
The doses to ask for are either 5mg daily low dose, or 2 x 20mg/week. They both give a continuous dosing background level, but the 20mg will give you a higher boost on those days, making them better for trying for erections on those days. Some men even manage to get both, some 20mg event dose, and some 5mg daily low dose for other days, although the 20mg takes some days to decay away to below the 5mg dosing level anyway.