What table notification is on? Is there a common/primary key in your notifcation table & u_platform table?
If you you need to try something as below
(function runMailScript(current, template, email, email_action, event) {
var nameis = new GlideRecord("u_custom"); //replace table name
nameis.addQuery("xyz", current.abc); //common field name to be replaced with abc here & xyz is field that is in custom table with same value as abc in current notification table
nameis query();
if(nameis.next()) {
template.print('Platform value '+nameis.u_platform_value.getDisplayValue());
}
})(current, template, email, email_action, event);
