Welcome to the forum, Andy.
F/G tells us to play a F major chord in "chord parts" (often this are RHY1, RHY2 and PAD parts), but to play a G in the bass part. While there are easy two note fingerings for some slash chords in AI Fingered, F/G is not among them. In that case it is always possible to "force" a bass note by octavating it. For F/G you may try:
G-A-C-F-G
As you see, A-C-F is a F major chord in 1st inversion, and a G as the lowest note together with an additional G one octave above will "force" the bass note to G. If you try that, you will find that the chord recognized will be displayed as Fadd9/G. This means that the chord parts won't play an F major but a Fadd9 instead.
Depending on the musical context you might be very happy with the Fadd9/G sound produced. After all there is nothing wrong with that, harmonically it is correct. On the other hand, those two chords sound distinctive and in some cases it is quite important IMHO. For example try 10cc's "I'm not in love" which starts with G/A - A. I think this progression just doesn't sound right when you play Gadd9/A - A, and the developer of AI Fingered was aware of that fact. So you may try the following "trick":
G-G#-A-C-F
As you can see and hear, this fingering produces a correct F/G chord. Of course, the G# note does not make any sense at all harmonically, so you could use this fingering only with the LEFT voice turned of.