The certificate warning you can experience in your web browser is the expected behavior when not using a free or third party certificate.
It will be displayed when a user tries to access your url using your HTTPS port.
This warning message can be displayed only one time (during fist connexion) or more depending their web browser settings.
When using HTTPS and not having a valid SSL certificate, your users need to click on "connect anyway/continue" from the "advanced/show more" button displayed from the warning message.
If you still do not want to have a certificate, then you can continue this way or your users can use your URL with HTTP instead of HTTPS.
For your infirmation, here is our free certificate procedure : https://docs.terminalserviceplus.com/tsplus/free-certificate-manager
And here for a third party certificate : https://docs.terminalserviceplus.com/tsplus/https-ssl-certificates-tutorial