Problem:

How to setup default value in the relationship field of child post form?

Solution:

You can follow our document to setup the child post form link, and pass an URL parameter to the child post form, and setup the default value:

https://toolset.com/course-lesson/selecting-parent-posts-when-using-forms-to-create-child-items/#creating-forms-when-a-parent-post-is-preselected

section "Creating Forms with a Parent Post Preselected"

Relevant Documentation:

Problem:

How to customize the message after user submit post form?

Solution:

You can try with the option "After visitors submit this form: Display a message instead of the form..."

Relevant Documentation: