trigger Workflow to update field when other field is changed on Create

trigger Workflow to update field when other field is changed on Create

Hi! I'm trying to write a function to update "project id" field in Deals module.
This id consists of such parts:
  1. year;
  2. group( which is q field value);
  3. index (will take the latest by the existing records);
I wrote the function, which works fine in a debugger. Now, I need to connect it to a workflow, where I have problems.
  1. if I select "Create" for "When do you want to execute this rule?" -> on create I don't have a value from group field yet
  2. if I select "Edit -> Specific field(s) gets modified" for "When do you want to execute this rule?" ->it will not work when a record is created.
How can I call a workflow when a filed value is changed(selected from a pick list) on Record Create? Or is there any other option, except a Workflow?