Learn English – Do you want me continuing

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".

Personally, I wouldn't choose either. The phrase 'as if' is normally reserved for hyperbolic hypotheticals rather than more ordinary ones - eg: "He was laughing so hard it looked as if he was choking on a piece of steak." While grammatically correct, both of your examples have a somewhat forced (unidiomatic) construction as well as being somewhat ambiguous with regard to past/past perfect state distinctions.

I would put things more simply as:

He looked like a beggar.

This is less ambiguous with regard to time and tense (It remains ambiguous as to actual knowledge of his true condition versus appearance). If however, you do wish to emphasize a distinction between past and past perfect states (I can't really imagine why in this instance, but in other practical examples, it might be more relevant), then some of the following constructions are more idiomatic:

He looked like he used to beg.

He looked like he had previously been a beggar.

This last is similar to your second option, but still just a little clumsy, I'd prefer:

He looked like a retired beggar.