I want to create a quick action to send an email. When attempting to assign an existing template to the predefined Email Template field I am left with a formula editor - how do I call up the email template for this predefined field value? I am doing this on a custom Object. As per Salesforce documentation I should get the Default Email Template field, but for me, this is not available.
There are actually 2 bugs in send email quick action functionality.
There is no way to set a template, its something which was uncooked and served. Only on Cases does it give you that option. (https://success.salesforce.com/answers?id=9063A000000e6pWQAQ)
You will still be able to add that quick-action/Salesforce Mobile and Lightning Experience Action on page layout, it would never be visible when you view them.(https://success.salesforce.com/answers?id=9063A000000DicUQAS)
If you still want send email, you are better off creating custom email component or Using Global Actions.