Looks like you have created this workflow just for mail sending and this WF is triggered using FM 'SAP_WAPI_START_WORKFLOW'. You can set the elements in WF container right before the trigger. Refer to usuage of 'SAP_WAPI_START_WORKFLOW' . Try to fill the table "LT_CONTAINER"
call function 'SAP_WAPI_START_WORKFLOW'
exporting
TASK = M_WF_ID
LANGUAGE = SY-LANGU
DO_COMMIT = 'X'
USER = SY-UNAME
importing
RETURN_CODE = L_SUBRC
WORKITEM_ID = WORKITEM_ID
NEW_STATUS = NEW_STATUS
tables
INPUT_CONTAINER = LT_CONTAINER
MESSAGE_LINES = MT_MESSAGE_LINES.