How do you handle salary negoatiations with third party recruiting?

A third party recruiting firm sent in my "agreed pay rate" for a company only after having me lowball my salary number before my actual interview. Can I just do normal negotiating post receiving an actual offer? Not a fan of how they pigeon hole you into "accepting" a low number before actually forwarding you to the hiring manager for the actual interview. Had the actual interview today and went great, hence my asking.