Problem:

I want to add the terms from project prompt post type to project submission post type on submitting project submission form. I am able to do that with url param as parent post id but I am unable to do that using $_POST("@relationship-field-slug.parent"). What am I missing? Is the slug for getting data from relationship field in a form different?

Solution:

Please try the PHP post variable:

$_POST['@project-prompt-to-project-submission_parent']
See PHP document:

Relevant Documentation:

https://www.php.net/manual/en/language.variables.external.php