You can't.
You can only choose a pre slanted font such as the itallic iso fonts..
Are you sure you not meaning rotate text or place text at an angle.
Try using the macro recorder
type in the key in aa=xx
where xx is the active angle
then key in ft=yy
where yy= font number
type in tx=zz
zz is the text size in master units
then review your macro to see the code sent to interpretor.