Hi Nicol,
In that case you have to create one custom search help in that provide process type field in that which you want to validate..
use that search help in your get_v method.
see this screen short you can come to know..
in get_v_fieldname you have to use this zsearch_help.
CREATE OBJECT rv_valuehelp_descriptor TYPE cl_bsp_wd_valuehelp_f4descr
EXPORTING
iv_help_id = 'ZSEARCH_HELP'
iv_help_id_kind = if_bsp_wd_valuehelp_f4descr=>help_id_kind_name
iv_input_mapping = lt_inmap
iv_output_mapping = lt_outmap
iv_trigger_submit = abap_true.
try this let me know if you have any queries..
Thanks & Regards,
Srinivas