Hi Jan,
thanks for your reply.
But I still can't figure out how to get a third button.
I navigate to Components - Phoca Download - Files
Now I add a new file for download.
Here are lots of parameters and I think I found the external link which you were talking about.
I entered something for testing. But I didn't see a third button.
And I don't see a field where I can give this new button a name.
There is still only the Download and Details Button.
But I need here a third button named Stream where I can enter a link.
Can you tell me how to configure the third button?
It should look like this:

(This picture is not real. I just made it in MS Paint. But it should look like this.)
Thanks for your help.