Type a sequence of characters, like: one, forward slash, three. Select all three characters. In the Character Styles panel, make a new style. Name it Frac-o-matic. Make sure it has no attributes in it so far. If necessary, click to Reset to Base to ensure it has no other attributes. Then go down the left side to OpenType Features. In that section you will see the choice of fraction. Click a check mark into it. OK the new style. Apply it to the three characters, if it isn't already, and you will see that it builds up the three (or more) separate characters into a built fraction. Most typefaces don't have more than a couple of pre-built vulgar (common) fraction characters. Your only necessary ingredient here is to be using an OpenType font.
Hope this helps,
Mike Witherell in Maryland