If you are on free plan, you can use Custom CSS editor to apply Myanmar font of your choice to the text you want to apply. You do not need to define font-face rule. Just need to assign HTML tag font-family declaration. If you are on Paid Plans, you can use CSS Hero editor to enter font name in font-family field. Myanmar font name will appear in the dropdown list though. You can enter the names described below. Currently the following Myanmar font names are supported.

Myanmar3

ZawgyiEasy

ZawgyiFormalHeading

ZawgyiInformalHeading

ZawgyiOne