Need to find INSTID_B for Notification (attachments)

I have a requirement to create attachments via RFC so they are attached to a quality notification.

I am going to use:

  CALL METHOD cl_binary_relation=>create_link
    EXPORTING
      is_object_a = ls_object_a
      is_object_b = ls_object_b
      ip_reltype  = 'ATTA'.

*  wa_folder_content-doc_id = im_instid_b.

  CALL FUNCTION 'SO_DOCUMENT_INSERT_API1'
    EXPORTING
      document_id                = wa_folder_content-doc_id
    IMPORTING
      document_data              = folder_content
    TABLES
      contents_hex               = contents_hex
      object_header              = object_header
      object_content             = object_content
*     attachment_list            = attachment_list
    EXCEPTIONS
      document_id_not_exist      = 1
      operation_no_authorization = 2
      x_error                    = 3
      OTHERS                     = 4.

  IF sy-subrc <> 0.
    MESSAGE ID sy-msgid TYPE sy-msgty NUMBER sy-msgno
            WITH sy-msgv1 sy-msgv2 sy-msgv3 sy-msgv4.
  ELSE.
    DESCRIBE TABLE contents_hex LINES lin.
    input_length = lin * sy-tleng.

  ENDIF.

I require IS_OBJECT_B which is also the document_id I pass to the document insert.

Now normally I could retrieve the instid_b using SRGBTBREL. This works for notifications that ALREADY HAVE ATTACHMENTS.

How do I find the instid_b link when there are no attachments? There must be a process to determine the document_id I require for so_document_insert_api1

Hopefully this makes sense.. if not please ask.