That form has nothing to do with a master password. It's the form used to save connection settings to a cluster.
Host name/address is the DNS name or IP address of the server hosting the cluster.
Port is the port that PostgreSQL instance is listening on, defaults to 5432.
Maintenance Database is just what it says, the maintenance database in that cluster, postgres is the default.
Username is the username of the role you want to use to connect to the cluster.
Password is the password for the role that you specified in username.
Save Password toggle is for if you want pgAdmin to remember the password for this connection. It's only if you do that the matter of whether or not you are using a Master Password matters.
Role is if you want to connect as one role but work as an alternate one.
The rest don't usually apply to most people, but may be applicable in certain situations.
And that error is from the first tab, it's the name of the connection in pgAdmin and it can't be left blank. I usually use the name of the server as a basis for that value.
I hope that that helps,
rik.