There is few ways to concatenate with blank space
Below is the classic example.
Rules:
  1. Make sure the variable field is in the correct length.

Below is the code snippet.

CONCATENATE sy-mandt wa_po-ebeln wa_po-ebelp INTO tp_tabkey RESPECTING BLANKS


Others way is using  class CL_ABAP_CONTAINER_UTILITIES.  If you want to set the space based on others data type. Below is the code snippet.


FORM insert_space  USING    p_ebeln TYPE ekko-ebeln
                   CHANGING tp_ebeln.

  CALL METHOD CL_ABAP_CONTAINER_UTILITIES=>FILL_CONTAINER_C
    EXPORTING
      IM_VALUE               = p_ebeln
    IMPORTING
      EX_CONTAINER           = tp_ebeln.
*    EXCEPTIONS
*      ILLEGAL_PARAMETER_TYPE = 1
*      others                 = 2
          .
  IF SY-SUBRC <> 0.
*   MESSAGE ID SY-MSGID TYPE SY-MSGTY NUMBER SY-MSGNO
*              WITH SY-MSGV1 SY-MSGV2 SY-MSGV3 SY-MSGV4.
  ENDIF.


ENDFORM. 

 Serkan AKKAVAK
Computer Engineer
serkurumsal@yandex.com