Dwalk,
The thing with augmented chords is that there are only four of them. Each note of an augmented chord is also that augmented chord.
For example...
Faug = F - A - C#
Aaug = A - C - E# = A - C# - F = Faug
C#aug = C# - E# - G## = C# - F - A = Faug
What this means is that
Faug = C#aug = Aaug. They all have exactly the same notes. The only difference is that the name of the chord will determine the bass note. Therefore if you want Faug/C#, then C#aug will give you the correct voicing.
In other words, there's no real reason to type Faug/C# because C#aug will do the job.
For what it's worth, when I entered F#aug/C#, BIAB gave me F#aug/Db which looks strange but it is correct since Db = C#.
As I said, though, simply use C#aug.
If you are using a US keyboard layout, simply type the following without spaces
C3aug and then press enter.
Regards,
Noel