Query regarding Authorization object

Hi friends,

In occupancy tab of t-code 'REBDRO' there is a field named as ' Vacreason ' . Now, the requirement is to restrict the user from changing the value other than what he/she is authorized to do so.

To meet the requirement, I have created a authorization object as 'Z_VAC' (through SU21) and given field names as VACREASON and ACTVT. Now, where I have to write the following code to restrict the user from changing the vacancy reason value? Is there any user-exit for it?

AUTHORITY-CHECK OBJECT 'Z_VAC'

ID 'VACREASON' FIELD '30'

ID 'ACTVT' FIELD '02'.

I will definetly reward points.

-Rishi