in that case you'd do this in the reference qualifier of the variable, not in a script include.

Your script include will also need to be updated.

var Firewall_IP = Class.create();
Firewall_IP.prototype = Object.extendsObject(AbstractAjaxProcessor, {
	PopulatIPsBasedonLoc: function(location) {
		var gr = new GlideRecord('ip_address');
		gr.addQuery('location', location);
		gr.query();
		var answer = [];
		while (gr.next()) {
			answer.push(gr.sys_id.toString());
		}
		return 'sys_idIN' + answer;
	},
	type: 'Firewall_IP'
});