Set field value to {tranid} after submit

We use a custom field that stores a common ID number across all elements of a transaction beginning with the sales order. This ID number is just a copy of the sales order ID number with the "PRFM" abbreviation (we call our sales orders proformas...) replaced with "CTG". Initially this was done by creating a sales order, saving it so that it's ID number would be generated and then opening it again and literally copying and pasting the number into our custom ID field and saving again. This is obviously not ideal.

I have now managed to set up a workflow that "on update" uses a "set field value" with the formula: REPLACE({tranid}, 'PRFM', 'CTG'}, but this still requires the sales order to be edited and saved again to seed the custom ID field so that it will carry through to the subsequent forms. I haven't figured out a way to do this "on create" without simply getting the value "To Be Generated" even if I use "set field value" after submit. Does anyone know of a way to pull the {tranid}, after it is generated, to our custom field without having