Value Help for (crm) activity category in a report

Hello experts,

I have to write a report where the transaction type of an activity is selectable by a value help. Does any data element exists where the F4 help get shown automatically for this purpose? Or what do I have to do to get the value help for all customizied activity categories? Do someone have a code snipped or something similar?

What I have tried is: PARAMETERS p_cat type CRMT_ACTIVITY_CATEGORY. --> But no value help gets shown. I´m an abap beginner... (I tried in an other case PARAMETERS: p_obj_id type crmt_object_id --> here the value help was shown automatically).

Please help me.

Thanks in advance and best regards

Tanja