How to attach email templates to existing email in d365

Hello Team,

in Standard D365 :

 project Management and accounting Module : while posting the project invoice proposal the email is triggering to the corresponding customer

my requirement is on that email i have to attach email message ( email template)

i have setup the email message   organisation administration>setup>Email templates> email messages

in the email message is not appearing on the report please help further on this  whether  it can possible only for X++ code or it can be possible also standard as well