ACS Push Notification to a specific User ID

Hello, I have a question for Cloud Push Notifications on IOS. I want to have the ability to send a push notification to a specific user from my app to their phone. I know the to_ids section is looking for the random letters and numbers generated in the acs database, so I want to make that value equal to the corresponding username who just signed up on my app. However, I do not know the proper syntax or approach to this problem. This is my thought process, although I know the to_ids won't accept the value from the textbox unless it is equal to a User Id. Any help would be appreciated.

var reportUser = Ti.UI.createTextField({
    borderStyle: Ti.UI.INPUT_BORDERSTYLE_ROUNDED,
    top: '50',
    left: '10',
    width: '300',
    height: '35',
    hintText: 'Enter username here'
});
 
win.add(reportUser);
 
var reportButton = Ti.UI.createButton({
    top: '150',
    width: '100',
    height: '25',
    title: 'Report',
    borderRadius:1,
    backgroundColor: '#68BDFF',
    font:{fontFamily:'Arial',fontWeight:'bold',fontSize:14}
});
 
win.add(reportButton);
 
reportButton.addEventListener('click', function(e)
{
    if(reportUser != '')
    {
     Cloud.PushNotifications.notify({
         channel: 'demo_alert',
         payload: "Look out behind you!",
         badge: "+1",         
         to_ids: 'reportUser.value'
     }, function (e) {
         if (e.success) {
             alert('Push notification sent');
         } else {
             alert('Error:\n' +
                 ((e.error && e.message) || JSON.stringify(e)));
         }
    });