If all you want is eighth notes on the snare drum, then don’t use Drummer at all. Create that specific part yourself.
Just create a drum track and do one of the following:
record a bar of yourself tapping eighth notes on the snare drum
record an empty bar, then tap the bar and choose “Edit” and in the drum editor, pick up the pen by sliding the pen slider at top left, and draw in your eighth notes on the snare drum line
… and then loop the bar you created to extend the part over the course of your song.
Drummer is for the times when you don’t want a specific part, but rather only want to provide some guidance like push or pull the feel, play more complex or more simply, and then get back a very typical drum part of kick, hats, snare, and fills. If you have a specific part in mind, you just create that yourself. Especially if it is a very atypical drum part like eighth notes on the snare.