How to add generic Usenet accounts?

First you should check if JD has a specified plugin for your usenet service by checking if you can find it under Settings -> Accounts -> Add

In this case, all you usually need to enter is your username/mail and password.
If you are unable to find your service there, enter "genericusenet" in the search field:
You should be able to obtain your usenet credentials and the other information in the account overview of the service you are using.
If in doubt, use the default values.

How to add generic Usenet accounts via myjdownloader/headless?

As you might have already seen, myjdownloader only allows you to enter a username and password.

There is a special format for generic usenet accounts so they can be added via myjdownloader:

nntps://username@exampleusenetserver.com:port/maxConnections

Use "nntp://" if your usenet server doesn't allow SSL.
If you do not know the port and/or max. connections, you can simply leave them out e.g.:

nntps://username@exampleusenetserver.com

Enter this string in the username and your password in the password field to add your account.