How to prefix a letter on PONumber based on Operating unit?

Dear Experts,

Our client is having 10 OU and for each OU we are using the same PO Document number which is 500000. To differentiate the PO based on OU we planned to prefix an alphabet on PO number.

Expectation:

Whenever the PO is created manually/through Autocreate form/Quote to PO the PO number should prefix with a letter which we hardcode.

Kindly put your thought point on this to achieve and we are trying for personalization but on first save its showing only PO number and on second save after any field value changes, the PO number is prefixed with hardcoded letter.