After you use ADD-NEW-INDEX(), you invoke the ADD-INDEX-FIELD() method once for each field in the new index. You pass the name of the index, the name of the field to add, and an optional third argument that evaluates to asc for ascending (the default) or desc for descending:
tt-handle:ADD-INDEX-FIELD( index-name-exp ,field-name-exp 
    [ , mode-exp ] ).

If there are multiple fields in the index, the order in which you invoke ADD-INDEX-FIELD() for the fields determines their order within the index.

This example adds a new unique index called SeqIndex to the temp-table and adds the Sequence field to it:
DEFINE VARIABLE hTT AS HANDLE NO-UNDO.
CREATE TEMP-TABLE hTT.
hTT:CREATE-LIKE("Customer","Name").
hTT:ADD-FIELDS-FROM("SalesRep","MonthQuota").
hTT:ADD-LIKE-FIELD("Area", "SalesRep.Region").
hTT:ADD-NEW-FIELD("Sequence", "INTEGER",0,"9999",1000).
hTT:ADD-LIKE-INDEX("CustNum","CustNum","Customer").
hTT:ADD-NEW-INDEX("SeqIndex", YES).
hTT:ADD-INDEX-FIELD("SeqIndex", "Sequence").