There are plenty of ways to accomplish that using B4A.
You could for example use the designer > add a 60x60 label > Give the label a corner of radius 60 > Text 'F' > Bold font > Font size about 32 or larger > Generate click event.
To add a shadow you can place the above label in a 60x60 panel > Give the panel a corner of radius 60 > Elevation of 5 > Generate Dim declaration.
You can also use a Button instead of a label.
You can also do the same without using the designer.
It just goes on and on and on...
What exactly are you going to use it for.