It is recommended to use a separate identity for each mailchuck account. This is how you create a new identity.
From the Bitmessage client, select the “Your identities” tab and click the “New” button:
In the “Label” field, type something that you will be able to identify, for example your desired @mailchuck.com email address:
You can leave the other options or modify them if you prefer. Then click OK.
It may take a couple of minutes for the address to be generated.