Is it possible to define input fields where the user is obliged to make an entry?

You can use the option "-required" in your InputField. This will display the field in the normal SAP manner as "required" field. But you still have to check in your InputScript that the user has indeed made an input. Example:

if not V[MM02_Material]
  Return "E: Please enter a material number" -statusline

Put such commands before the first "Screen" command of your InputScript.