Credentials format to login into Windows Domain?
Generally speaking logging onto a windows domain from a Mac takes the form of:

[DOMAIN]\[USERNAME]
[PASSWORD]


so if I were a user in the Microsoft Active Directory domain with the username of johnny.appleseed and my password was 1234qWer, that would look like

microsoft\johnny.appleseed
1234qWer


Note that the username and the password are not case sensitive I generally use all lowercase for simplicity's sake.

Note that you must have a username assigned to you in the domain with the necessary permissions on the target server for that particular login/username. Without that it will never work.

If the target (Windows) server is not domain-joined then you would use the name of the computer (sometimes called the NETBIOS name) on the network in place of the domain. But you must have a username, password and the appropriate permissions created on that server for the login to work.

I also find that when connecting to a PC via the GO > Connect to server dialog it sometimes helps to use the IP address of the target server, if you are not connecting using the server's name. Examples below.

smb://192.168.1.1/share
smb://bigserver/share