Need BADI for REASON field on CRMD_ORDER

Hi CRM experts.

I am new to CRM and have the following assignment. On transaction CRMD_ORDER for a certain transaction type I have to call my authorisation object on the REASON list and eliminate certain reasons in this dropdown list according to this object.

We thought that BADI CRM_ORDER_FIELDCHECK would do the trick as we already have code there that make the REASON list not changeable. My problem is I need to loop through this actual reason list in order to call the authorisation object and on this BADI only the FIELD name is visable and not the contents of the field. Can someone assist please?