Populate text box

SOLVED

Hello!

We would like to have a textbox on Enter Sales Orders and Quotes that populates once a customer, and subsequently a salesperson, is selected. 

I've added the box and binded it to the appropriate field, but it doesn't populate until we save and close the record and go back in. 

How can we make it populate once the salesperson field is populated on the form? 

Thank you.