Maingun Domain name

I have some issues here, the image above is one and the other is im trying to fill the fields fom WordPress but there are a few things that im not sure about like:

<span style=”color: rgb(29, 35, 39); font-size: 14px; font-weight: 600; white-space-collapse: collapse; background-color: rgb(240, 240, 241);”>Mailgun Domain Name</span>?where is this?

Thanks a lot.

The page I need help with: [log in to see the link]