Different Network Behavior in adding player between editor play and built

The NetworkManagerHUD as offered by Unity always asked me "Client Ready?" after I set up the server. But only in Built mode.

If I play it on the editor the client on the server gets created automatically. How can I get rid of that?

The final goal is to use my own NetworkManagerHUD script. But it seems that this "Client Ready?" issue still causes problems using a script.

Any idea?

Thank you in advance