Purchases: Ability to set own purchase order number sequence (including alphabetical characters)

I want a letter and number sequence for both my purchase orders and a different one for my sales orders. Nowhere does it give me the option to change the automatic sequencing or does it allow for letters to be entered in. I understand that if I enter a certain number and save the purchase order or sales order it will then generate from that number onwards. However I want to use letters as well, so this is still not helpful. Being able to "select starting sequence" would be very helpful. Thank you.

"Being able to manually set a purchase order number sequence"