Okey so this is what I want to do:
I have one signup form on which people can offer help for an activity. For example "Learn how to draw". This title will be saved as the "activity_title" in the database.
After that there is another page on which all those offers are displayed by the [pdb_list] shortcode and people can sign up for this offers. For that I added a signup button to a custom pdb_list template which redirects the people who want to participate to another signup form.
And now in this signup form for participants they have to enter last name, first name, etc. and need to choose for which activity they want to signup. To select an activity I want to show a dropdown field which contains all saved records from the field "activity_title" as the dropdown values.
Hope this explains my thoughts.