If you need to use a Dedicated Administrator Connection (DAC) via SSMS, you can’t just use the Connect Object Explorer, if you try you will get the following error

Dedicated administrator connections are not supported via SSMS as it establishes multiple connections by design. (Microsoft.SqlServer.Management.SqlStudio.Explorer)

So what do you do? You can connect from within SSMS, you need to click on the Database Engine Query icon

Prefix the servername with Admin: Instead of (local), you would do Admin:(local)

Another way to connect is by using sqlcmd with the command-line option A
Here is an example with a username and password

sqlcmd -S ServerName -U sa -P Password –A

You can also use a trusted connection

sqlcmd -S (local) -E -A

If you want to know more about DAC, check out Using a Dedicated Administrator Connection