I have an InputField that refers to the SAP field "tax code" using the option techname="LFB1-QSSKZ". But no search help is shown when I press F4. Why?
I would also like to restrict the search help to country code "US", if possible.
There is no search help defined for the field "LFB1-QSSKZ" in SAP data dictionary (see transaction SE11). In this case GuiXT is not able to display a search help.

So you have to specify the search help name directly:

.... SearchHelp="H_T059Q"

Since this search help exports two fields (country and tax code), GuiXT takes only the first one. Specify shname= to get the second field value instead:

SearchHelp="H_T059Q" shname="QSSKZ"

Finally, if you want to restrict the search help to country code "US":

... SearchHelp="H_T059Q" shname="QSSKZ" shselname1="LAND1" shselvalue1="US"