I want javascript which can help me to get value from a field and put it into another field.

ex: I have additional Field shopcode and shopname, where I want whenever user input information into these 2 fields, javascript run and get value from these 2 fields and put it in subject

shopcode: 123456

shopname: XYZ Company

subject should be: request for support from 123456 XYZ Company

thank you !