It isn’t clear to me that this custom prompt is something which is available to the user, as opposed to the site admin. I can’t seem to find it, anyhow: here on Meta as a user I am offered just four choices, one of which is “Proofread”.
I think I might in the same boat as @jidanni - I am approaching this as a user, not expert in the featureset that’s offered. Hopefully that position is a useful one, for constructive feedback.