Under
Iowa workers compensation law, the employer normally chooses the doctor through whom workers' compensation benefits are delivered.
It may be that the agency is more interested in what could be deemed an "IME" or "Independent Medical Examination" - where their doctor reviews your situation and issues an opinion as to whether or not your injury is work-related (or subject to benefits) - as opposed to providing treatment.
You may benefit from discussing your situation with an Iowa workers' compensation lawyer in advance of the examination.