For form entry to set the print parameters u have to call one perform.

For form entry to set the print parameters u have to call one perform befor calling the smartform.

  PERFORM set_print_param USING    lds_addr_key
                              CHANGING lds_control_param
                                       lds_composer_param
                                       lds_recipient
                                       lds_sender
                                       prf_return_code.

**................. To call the smart form......................*

  CALL FUNCTION gdf_mod_name
    EXPORTING
*   ARCHIVE_INDEX              =
*   ARCHIVE_INDEX_TAB          =
*   ARCHIVE_PARAMETERS         =
    control_parameters         = lds_control_param
*   MAIL_APPL_OBJ              =
    mail_recipient             = lds_recipient
    mail_sender                = lds_sender
    output_options             = lds_composer_param
    user_settings              = lcf_blank
      gds_header                 = gds_header
      gdf_ship_adrnr             = gdf_ship_adrnr
      gdf_plant_adrnr            = gdf_plant_adrnr
      gdf_pstyv                  = gdf_pstyv
      gdf_plantname              = gdf_plantname
      gdf_vbeln                  = gdf_ekkn_vbeln
* IMPORTING
*   DOCUMENT_OUTPUT_INFO       =
*   JOB_OUTPUT_INFO            =
*   JOB_OUTPUT_OPTIONS         =
    TABLES
      gdt_item                   = gdt_item
* EXCEPTIONS
*   FORMATTING_ERROR           = 1
*   INTERNAL_ERROR             = 2
*   SEND_ERROR                 = 3
*   USER_CANCELED              = 4
*   OTHERS                     = 5
            .
  IF sy-subrc <> 0.
* MESSAGE ID SY-MSGID TYPE SY-MSGTY NUMBER SY-MSGNO
*         WITH SY-MSGV1 SY-MSGV2 SY-MSGV3 SY-MSGV4.
  ENDIF.
*****************************************************************
  FORM set_print_param  USING   prs_addr_key       TYPE addr_key
                     CHANGING prs_control_param  TYPE ssfctrlop
                              prs_composer_param TYPE ssfcompop
                              prs_recipient      TYPE swotobjid
                              prs_sender         TYPE swotobjid
                              prf_return         TYPE sy-subrc.
*Local Declarations.
  DATA: lds_itcpo     TYPE itcpo.
  DATA: ldf_repid     TYPE sy-repid.
  DATA: ldf_device    TYPE tddevice.
*Program Name
  ldf_repid = sy-repid.
*Local Constants.
  CONSTANTS: lcf_x(1) VALUE 'X'.
*Smartform Conditions.
  CALL FUNCTION 'WFMC_PREPARE_SMART_FORM'
    EXPORTING
      pi_nast             =  nast
     pi_addr_key         = prs_addr_key
*     PI_COUNTRY          =
      pi_repid            = ldf_repid
*     PI_SCREEN           = ' '
   IMPORTING
     pe_returncode       = prf_return
     pe_itcpo            = lds_itcpo
     pe_device           = ldf_device
     pe_recipient        = prs_recipient
     pe_sender           = prs_sender            .
  IF prf_return = 0.
*Setting up the Display of Smartform.
    MOVE-CORRESPONDING lds_itcpo TO prs_composer_param.
    prs_control_param-device      = ldf_device.
    prs_control_param-no_dialog   = lcf_x.
    prs_control_param-preview     = gdf_xscreen.
    prs_control_param-getotf      = lds_itcpo-tdgetotf.
    prs_control_param-langu       = nast-spras.
  ENDIF.


ENDFORM.                    " SET_PRINT_PARAM