Rasalin,
You can provide F4 help in PROCESS ON VALUE REQUEST event of the generated screen for the respective field.
to update the MAKTX field based on entered material, you write a module in PAI of generated screen and update the respective field value.
Through TMG event you can validate the input data, but change and updating any field value may not work while saving the data in DB table.
Reagrds,
Praveer.