Hi,
After reading value from CL_MDG_BS_MAT_ASSIST_UI=>GT_CUSTOM_URL_PARAM, try changing CS_DATA in GET_DATA method as below:
FIELD-SYMBOLS: <lv_extwg> TYPE extwg.
ASSIGN COMPONENT 'EXTWG' OF STRUCTURE cs_data TO <lv_extwg>.
<lv_extwg> = "Value for field EXTWG from CL_MDG_BS_MAT_ASSIST_UI=>GT_CUSTOM_URL_PARAM