But I'm not sure about to fill in blank at agency owner agreement document.
I think that your example sentence is close enough that we can fix it up before I tell you how I would say it.
You are missing something before "to fill" to be the subject of the infinitive clause and you should put a determiner in front of "blank", like "the" or "this" if you are talking about a specific blank.
But I'm not sure about _____ to fill in the blank at agency owner agreement document.
You could say:
I'm not sure about how to fill in the blank at agency owner agreement document.
I'm not sure about what to fill in the blank at agency owner agreement document.
I would probably express the idea like this:
I'm not sure what I should put in the blank at "Agency Owner Agreement Document".