Requirement to calculate a field value on the process form based on the input from request form.

I have installed SSH OOB connector and need to do some customizations for data input.

I need to calculate the “User_login” field value based on the value of one more field “primary group”. Basically once  a resource is requested from catalogue and the request form is displayed, user will select the “primary group” from look up. “User_login” field is calculated based on the first_name, last_name  and the “primary group”. I can calculate the field and populate the “User_login” on the same request form or I can send it directly to the underlying process form(ud_ssh)