Create mask for rg C#

Question

I wonder if there is any way I can make appear the mask made through the code in my form? And in which way it would be, is not appearing in my form the way I am doing. In case I do not know if I would be putting in the correct position.

if (mtbRG.Text.Length == 9)

{
   mtbRG.Mask = "00.000.000-0";
}