Dynamically populating multiselect dropdowns

Hi

Is it possible to add multiple default values? Like, I have a multiselect dropdown with values:

1=Argentina

2=China

3=Serbia

I would like to preselect values 2 and 3 (China and Serbia). I tried to add 2,3 to the default field, but it doesn't work when I add a comma separated list of values. It works when I only add 1 value.

Is it not possible?