You must "transport" the dynpro field value yourself as explained by SAP in the ABAP doc for AT SELECTION-SCREEN ON VALUE-REQUEST :
"The events for the field help and input help do not transport data between the selection screen and the ABAP program. As with general screens, suitable function modules such as RS_SELECTIONSCREEN_READ and RS_SELECTIONSCREEN_UPDATE must be used for this. The parameters and selection criteria changed for the input help are transported to the selection screen."