Answer: You have to Login/Register, after login go to "Settings" page, now you can see "API settings". you can get the API/Token.
Yes, You can check the no of subscribed users list in "Users" menu
Via plugin after entering the API key. Check your website front-end, there is a popup notification top left side https://prnt.sc/qocwln. When user will allow it, subscribers will add automatically.
Make sure you have registered Users with the API. Now go to campaign page, Create a new campaign to send the message.