This is an easy one. You don't contact an agent until you get rid of that prologue and provide the reader with the necessary information in a more palatable way.
I don't know you or what kind of fiction you write, but I hope you'll trust me when I tell you that you do not need an info dump to explain how the world you've created works, the lore and legends of a culture vital to the story, the expectations of society in regard to people like your character, the political situation that's behind the war your character seeks to end, or anything else. Agents will reject you--which often means complete silence, not "no thanks"--as soon as they see it's an info dump.
Working that information into the story is absolutely vital. There are many methods. Science fiction has to do it all the time, so reading a sampling might show you how different authors manage.
One time-honored means is to introduce a traveler and have people explain things to him/her.
Meanwhile, back at your question. Yes, you send the amount of sample pages the agent asks for starting with the beginning of the book, whether it's a prologue or not.