If you want to add an index to a temp-table that is not derived from an index in another table, use the ADD-NEW-INDEX() method:
tt-handle:ADD-NEW-INDEX( index-name-exp 
    [, is-unique 
    [, is-primary
    [, is-word-index ] ] ] ).

You supply the index name and up to three optional logical values that indicate whether the new index has enforcement of unique values, whether it is the primary index of the temp-table, and whether it is a word-index. (A word index is a special index type on a CHARACTER field that allows the AVM to retrieve records based on any word the field contains.)