Most if not all congresspeople should have a section on their website for constituent requests and one option should be immigration help, USCIS expedites, something like that.
I submitted a form through the website with basic information about my expedite request, then the office followed up asking for documentation of evidence to support the request and a signed form giving them access to my personal information for the purposes of the request. They then notified me they had submitted the request, and a month later they emailed me again saying the request was accepted by USCIS. A few days after that my application was approved.