Small question about Grab ability?

[Daermoth]

This is just a small thing that I'm pretty sure I know how it works but would like to double check.

Basically, when the monster uses the grab ability, can it roll CMB and take the -20 after seeing the result, or do you have to declare the use of the -20 before you make the roll?

[avr]

It's before, like other attack modifiers.