Passing multiple ids to a single referenced query field

Hi, hope someone can help me understand this better. I have a entity referenced field on my webform, the select option. I have set it to have multiple values. I am trying to pass more than one content id to this via a url query. Currently my query works for one value, using ?refer=1 for example. I am trying to pass more than one id. If I set the default value of that field to 1,2 for example both values show in my select list, but trying to do this via a ulr?refer=1,2 doesnt. Is this possible? Thank you in advance.