If you wanted to test your mail server, you can use the included console command for testing emails, for example:
./console core:test-email emailHere@example.com
As a workaround for now you can use the API method to create new users. Since there isn’t an example, I wrote one that can be used below.
If you wanted to simply use your browser, then you can do a simple GET request by replacing the values below with the ones you want to set for your user. It’s important to remember that the email and password both need to be URL Encoded.
https://your-matomo-url.com/index.php?module=API&method=UsersManager.addUser&userLogin=UserNameHere&email=url-encoded-email%40example.com&password=URL-encoded-password&initialIdSite=1&token_auth=your-token-auth-here
However, since this is a GET request, it will be stored in both your browsers history as well as the web server access logs, so you could instead send a POST request using something such as cURL:
curl -X POST -d 'module=API&method=UsersManager.addUser&userLogin=UserNameHere&email=url-encoded-email%40example.com&password=URL-encoded-password&initialIdSite=1&token_auth=your-token-auth-here' https://your-matomo-url.com/index.php