We just added the figured bass notation. You will find it under the Text toolbar.
Figured bass notation is a way to notate chord, by specifying degrees that are to be added to a root note. A major chord would be written 5/3, as we are adding the 5th and the 3rd degrees on top of the root. On Flat.io, you need to first input 5, press enter, then input 3
As the 5/3 is quite common, we usually left those out. Then the first inversion of the triad is written 6/3, and the second inversion 6/4.
There are many ways to alter the degrees. We can either to that by adding an accidental (# for sharp, b for flat, n for natural). If we just have the accidental, the 3rd degree is assumed.
Another way to alter degrees is to have a slash/back-slash over the degree. On Flat you can do this by inputting the slash/backslash after the degree.
You can learn more about using Figured Bass notation on Flat with our dedidcated help article.