French Work Permit - Is This Legit? (Paris)

American here! New to InterNations and I hope this is a proper place to post this question.

A Portage Salarial (a.k.a. "Umbrella" Company) in France has offered to get me a work permit in France so I may apply for my long-stay 12 month visa in USA. Their verbage has been very confusing, they only say they will "give me the necessary Cerfa form and documents I need for the USA embassy".

Everything I have read online says the normal procedure involves the company applying on my behalf to the French authorities, and and then I will receive an invitation to come to the US embassy to complete the process.

I don't see anything about this CERFA form they are offering me to be a valid start to my obtaining a work-permit and then long-stay visa and am worried they are misinforming me.

No one via phone at any french embassy in USA will answer my question, nor can I seem to find the answer online. Any help very much appreciated!