What I don't get is what link to put in the button.
I have created a form and it works as expected when I have it on the parent page content template, but it would be nicer to only have a button and not the full form. It doesn't work if I have the form on a page and link to the page.
I guess it's very easy and obvious when I get it, but so far all the documentation I've read have not given me the solution.
Thanks in advance!
You will see a short description on how to have the button.
Essentially you will need to create a content template and insert the form onto that content template.
Then go back to the Parent post content template and insert a Field and Text Block and use the Forms button to insert the Child Post Link. See My Screenshot.
You will need to customize the link with a bit of custom css so it appears like a button.
Also yes you do need the content because this is what generates the dynamic page for the form. Adding the form to a regular page doesn't work, unless you manually create a link to it and pass the correct parent parameter value into the URL of the link.