Adding A Text Box For A Manual Payment ?

[QuangL]

Hu guys, 

Just wondering if there is any method of adding a "Required" customer filled text box for a Manual Payment in the checkout process ? Im using a Shopify store for one of wholesalers who use purchase order #'s. They want to be able to enter that PO# into a text box on the checkout page. Is this possible to do? My business really need this function to compare all purchases statements end of month. Any help or direction would be much appreciated.