Result

https://buttondown.email/morse?as_embed=true

Creating the form

First get your Buttondown embed url at https://buttondown.email/share#iframe

Then paste the iframe code in an /embed block

How do I center text above the form?

You can create 3 Notion columns and put the text in the center one.

To create an empty column you can drag an empty line at the beginning or end of an existing line