How to assign default values ​​for an edit form?
You need to check that value returned by _get_this_domain_id() function is in the array key used to create #options for form element ['field_genric_domains'] .

Example :

If _get_domaine_options() function returns array as :

array(1=>'India',2=>'USA');

Then _get_this_domain_id() should return key either 1 or 2 .That will set the default value depending on the array key .