Learn English – Phrase for not making a big deal out of something that is a big deal

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".

There are several ways of expressing something quickly happening right after something else, in decreasing order of quickness

immediately
We entered the house and immediately it started to rain.

suddenly
There was a loud thunder clap and suddenly it started to rain.

just then
The temperature dropped and just then the wind started to blow harder.

then
First we were outside, then we walked around, then it got colder, then the wind started blowing, then we ran to the house, then it stared to rain.

If more than one thing is happening at the same time then

while
We are inside the house while it was raining.

at the same time
We were inside the house and at the same time the dog was outside getting wet.

during
It was raining during the day time.

all the while
All the while it was raining, we watched TV.