You should use maintenance view to display name corresponding to customer number. With use of Maintenance view, you can maintain related data in several tables. Check out this link to know more about Maintenance view:
To perform a check for the mandatory, edit the generated screen and in screen flow logic you should add a module to check if the required fields are filled or not. You can also set the field possible to mandatory in the screen field options, however this is not recommended as it will show it as mandatory even for empty lines.