We don't use rates, but looking at the PROJ object in https://developer.adobe.com/workfront/api-explorer/, there's a collection on it for "rates". I imagine you'd do a POST to the PROJ/{id} object and set the rates in the body.

I recommend setting a project just like you want it and then use a GET PROJ/{{id}}?fields=rates to see what the structure is so you know how to match the body. There's going to be a bit of playing with it until you get it just right.