Overwrite a field value

Hi,

I have two tables name "Accounts" and "Payroll", there are two field common to both the table i.e. "Accounts_ID" and "Payroll_ID".

The field "Accounts_ID" needs to be manually entered by the user, whereas "Payroll_ID" is picking the value that the "Accounts_ID" is having.

I have a scenario where if "Accounts_ID" is empty then "Payroll_ID" picks the value from "Customer" table for field name "Customer_ID" and if "Accounts_ID" is not empty then overwrite the value of "Payroll_ID" as "Accounts_ID" value.

What method should I enter and should I enter under the form level or table?

Please be kind as I am new to AX.