Adding a visible label to a custom shelf Button

Can someone tell me how to add a visible label to a custom shelf Button ?

imageOverlayLabel should do the trick.
I’d recommend tooltips for longer descriptions, although you may need to enable toolips on hover in the Maya preferences. Sometimes its disabled by default?

https://help.autodesk.com/cloudhelp/2018/CHS/Maya-Tech-Docs/Commands/shelfButton.html#flagimageOverlayLabel