What is needed?
To be able to access the UI (dashboard) using HTTPS protocol you need to provide the Proxy Manager with an SSL certificate and private key for your domain.
The certificate and private key should be generated specifically for your domain and installed on the server. Usually it's done through the server and domain provider
How to do it?
Run Proxy Manager along with environment variables SSL_CERT and SSL_KEY pointing to the according certificate files. Example command to run in the terminal:
SSL_CERT=/path/to/ca.crt SSL_KEY=/path/to/ca.key pmgr
Once it is started you can access the UI using https://your-domain.com:22999/