Special search help: For an InputField "Code group" I cannot find the right search help. In the SAP transaction QM02 (Change notification) a tree list is displayed as search help. How can I invoke this in GuiXT?

It appears that this is not a standard SAP searchhelp defined in SE11 but is implemented directly in transaction QM02, using the function module QPK1_GP_CODE_PICKUP.
In such cases you need to implement an ABAP program that calls this function module. The InputField definition is as follows:

InputField (10,18) "Codegroup" (10,30) size=8 name=cdgr

searchhelp="exit.ZZGPCODE.GPCODE"

And here is the source code for your ABAP program:

PROGRAM  ZZGPCODE.

FORM GPCODE

    tables

       sel    "table with  shselname1=, shselvalue1=,

       dest   "table with  shname1=, shdest1

     using invalue

       changing selvalue.

Data: QPK1CD like QPK1CD.

  CALL FUNCTION 'QPK1_GP_CODE_PICKUP'

    EXPORTING

      I_KATALOGART           = '1'

      I_CODEGRUPPE           = '*'

      I_CODE                 = '*'

    IMPORTING

      E_QPK1CD               = QPK1CD

    EXCEPTIONS

      NO_MATCH_IN_RANGE      = 01

      NO_USER_SELECTION      = 02

      NO_SELECTION_SPECIFIED = 03.

       if sy-subrc = 0.

         selvalue = QPK1CD-CODE.

      endif.

ENDFORM.