Lebanese International University Codes Edition Paper

Hello, I am trying to populate a field depending on the selection of another field in this form.

So the website consists of:

Vehicles List page (VEHICLE NAME, other details, VEHICLE AMOUNT, other details)

Bookings List page (customer name, SELECT VEHICLE, other details, VEHICLE AMOUNT, other details)

I want after selecting a specific vehicle in the SELECT VEHICLE dropdown in bookings page, that the VEHICLE AMOUNT gets filled automatically depending on the selected vehicle.

Notes: database names(v_amount in vehicles page and t_amount in bookings page)

How can I edit the codes? Kindly find attached the screenshots and the codes used. The whole source-code will be sent.