In addition to my above update
Example
i_fcat-fieldname = 'KUNNR_NM'.
i_fcat-seltext_l = 'Sold To Party.'.
i_fcat-outputlen = '00035'.
i_fcat-key = 'X'.
i_fcat-key_sel = 'X'.
append i_fcat to i_fieldcat.
clear i_fcat.