Find your email defaults at Newsletter Glue > Settings. You will automatically be taken to the Connections tab. At the end of the tab, you will find the Email default section. By default the section will be disabled but It will be automatically enabled after you connect with ESP. The section will display data (if available) from your ESP.

Email defaults

These are the defaults you'll use when sending any post as a newsletter. 

The purpose of these settings is to make it quicker for you to send a post as a newsletter. They show up inside each new blog post you write.

Once you've added your defaults, you can Add new post, scroll all the way down and check the send as newsletter box. There, you'll see your email defaults auto-filled for you.