What have you done with Phoenix so far?
For example, it would be helpful if you are already familiar with Building Forms, Programming Phoenix ≥ 1.4 p.60. Fundamentally with HTML the create action is a result of a form being submitted.
resources "/contracts", ContractController
creates among others a POST /contracts HelloWeb.ContractController :create route which typically the form inside the page loaded with /contracts/new submits to. When that submit happens, the information inside the form is sent to the server as form data. The form data is essentially a list of key/value pairs - Phoenix turns those key/value pairs into a single Elixir Map. That map is supplied as the second (params) parameter to the create action inside the ContractController.
Given the above the simplest approach is to represent the retailers as radio buttons (the value being the retailer_id) inside a form. That way the retailer_id should appear in create’s params map value. The current user should be available in the (first) conn argument (conn.assigns.current_user) giving you the necessary information.
Don’t forget to redirect at the end to let the user know whether the “contract create” failed or succeeded.
