Hi Minda, you could use the method Mark suggested above, but using an automation rather than a form rule. So the automation will listen for new records to be created and move the field value from your formula - text field to a non-formula text field. That way it will work with your Copy Master and Detail Records button.
The reason why you generally cannot set a formula as a key field, by the way, is that every record needs to have a unique value for the key field, but the value of a formula field can change at any time.
