If you need to use a general signature every time you send an email to your customer, for example for the support service, but you would like to include the user name in that signature based on who is sending the email, the expression you need to copy in the content of the template is: ${user.name} . 

I was not able to get this expression using the Dynamic Placeholder Generator, but actually you only need to copy the mentioned expression, not needed to generate it.