Hello Klas,
Thanks for your help. Finally I could figure it out:
Here an example of the Dynamic Select Field SQL-Statement with URL-parameter I use in the Form Type:
SELECT a.id AS value, a.title AS text FROM #__content AS a
INNER JOIN #__cck_store_form_xxxxxx AS b ON (b.id=a.id)
WHERE a.catid = 11
AND a.state = 1
AND b.myparamfield = $uri->get('myparam');
The value of myparam is another dynamic select field in the Search section of a List & Search, from which the Admin User is entering the Form & Content. There parameter is sent under List -> Nr. 2 -> Link select Form and open the Popup by clicking on the +-symbol next to "Form". - There in the field "Custom variables" add:
myparam=$cck->getValue('myparam')
After doing all this I noticed that the Save Button did not work any longer - as I described in my first post of this thread.
I added a Seblod Button Free which points to the form and which I added the the parameter, similar to List -> Nr. 2 -> Link.
Hope all this makes sense and is of help for someone.