Create Server Password StarBound

Setting a general server password is not possible.
Instead, you need to first create a “user account” in the starbound_server.config file.
To be able to edit it, the server must be in Advanced Mode.

Open your server for editing by clicking on its name in the server list.
Alternatively, you can click on the gray gear icon.

Then, click on starbound_server.config on the left and scroll to:
"allowAnonymousConnections" : true,
Change the entry as follows:
"allowAnonymousConnections" : false,

With this change, players will now have to enter a username and password when connecting.
For these players, an entry must be created in the starbound_server.config file.
The entry for a user looks like this:

      "serverUsers" : {
         "UserName" : {
             "admin" : false,
             "password" : "YourPW"
             }
      },

Replace UserName with the name of the user you want to allow to join the server.
The entry could look like this:

      "serverUsers" : {
         "Urizen" : {
             "admin" : false,
             "password" : "TestPW"
             }
      },

And with multiple users:

      "serverUsers" : {
         "Max" : {
             "admin" : false,
             "password" : "TestPW1"
             },
         "Mix" : {
             "admin" : false,
             "password" : "TestPW2"
             },
         "Mox" : {
             "admin" : false,
             "password" : "TestPW3"
             }
      },

Then click on Save and restart your server.

Note: Admins are created in a similar way.
How to do that is described here: How to Become an Admin