What do I roll to cast a summon with pre-determined base attack bonus?

I am playing a warlock in my current campaign and I selected earthen grasp for one of the invocations. Earthen grasp comes with its own base attack bonus:

Earthen Grasp:

Treat the arm as a Medium creature, with a base attack bonus equal to your caster level and a Strength of 14 +2 per three caster levels (16 at 3rd level, 18 at 6th level, and so on).

Looking at this, it doesn’t seem to require any sort of roll since the strength bonus is always pre-determined. In that case, what would I need to roll in order to summon/cast an earthen grasp?