A doit is an effect often performed by brass instruments in which the player produces a kind of upward glissando after attacking the indicated note, producing the opposite of a “fall-off” effect. These are usually added as articulations. You can create the word “doit” as an expression (see Expressions and Slurs for information on creating an adjustable line in the score, see Shape Designer), and the line itself as a second expression (in this case, a Shape Expression). For instructions on creating a custom shape, see Creating a shape.
You can also purchase special music fonts for use with Finale, some of which contain a selection of useful jazz symbols (including doits). One such font, Broadway Copyist, is included with Finale.