How to pre-populate a caldera form?
Actually, the process is easy:

In the Caldera form &gt; tab Builder, add an hidden field with the value {get:field_name} (replace field_name by the desired value)
In the Caldera form &gt; tab Process, add a CiviCRM process and match the needed field with the one from Caldera with the magic field syntax, so here %field_name%. (In my case, 'employee of' matches with '%organization_name%')
create an url to the page with the form: www.domain.com/form/ and add ?field_name=YOURVALUE so it shows up like www.domain.com/form/?organization_name=YOURVALUE. You can also add several variables using '&amp;' between them.
Add the User registration Process with password creation
Send the email from inside CiviCRM using such a syntax: https://domain.com/form/?organization_name={contact.organization_name}

When processed, the new user gets a CiviCRM membership as a relation to the organization and can access specified content of the WordPress site!
Source: Caldera form documentation