On MUI 3.x this attribute can only be specified during initialization (applicability is just I on MUI 3.x).
If MUI 4 and MUI Royale 1.2 are available, then you can also use mui.Set() and mui.Get() with this attribute (ISG applicability).
Furthermore, if MUI 4 and MUI Royale 1.2 are available the string you specify here can use text formatting codes. See Text formatting codes for details.