Why don't the F4 values appear when we press F4 on this input
are many SAP fields that do not have a standard searchhelp defined in the
repository (transaction SE11). The F4 search help in such cases is directly
implemented in the SAP application; in most cases the application calls an ABAP
need to implement an ABAP exit program for the GUIXT F4 searchhelp:
order to obtain the name of the function module that implements the searchhelp,
use Tips&Tricks Nr. 18. The called function is shown in the ABAP "call
stack" when the searchhelp is opened up.
your particular case you will find that this is the function
the exit as described in the "InputField" keyword documentation.
See also Q&A InputAssistant Nr. 317 for an example.