In EasyEDA is is possible to "Add solder mask" but this is a confusion description. In fact what happens is that it adds an aperture in the solder mask to expose the area underneath it.
It would be clearer to describe the tool as:
"Add Solder Mask aperture"
or;
"Add Solder Mask window"
Chrome
71.0.3578.99
AndroidOS
8.1.0