Hey @eman​! Daniel from Customer Support Engineering here 🔧 

You can insert a User ID, User email, or any other type of data associated with the user by clicking the {...} button that appears to the right of the editor while you're writing the content of your Message. Once you do this, you should see a menu pop up with those fields listed in it - just click that field and it'll be auto-populated depending on the attribute's value for the customer that receives the message👇

I hope this helps! 😁