Add an auto-enter calculation: TextFormatRemove ( self ) and clear the "do not replace existing value..." check box. Then you can select the field on your layout and specify a font for it. Note that this is an expression that you can copy and paste into the specify calculation box for multiple fields if you need it.
How would I specify the font for it?
Enter layout mode, select the field object, then use either the formatting tool bar (click Aa) or the inspector's appearance tab to select a font for the field.