If the sample you want to add to your casting is already in your database and its GDPR processing has already been done, you can find it in the “Casting” tab. From there, you can add it to a pre-existing casting list, or create a new one. You can also add an existing sample to your casting from the desired casting page, in “Saved Castings”.
If your sample is not part of your database yet, first make sure that the voice actor linked to it already is. Note that you will not be able to add the sample to any casting list before its GDPR processing has been completed first.