Set up A Record of the Domain Name
Set up an A Record for your domain (details vary depending on your domain service provider) so that the domain points to the host's IP where TurboFTP Server runs on. (The Domain here means your Internet domain name.)
Stop TurboFTP Server
Make sure TurboFTP Server has stopped. To stop the service, go to Tools -> Local Service Manager.
Install IIS on Windows Server
Install IIS by adding a Web Server (IIS) role. Then launch Internet Information Services (IIS) Manager, right-click on "Default Web Site," and select "Edit Bindings". Click on Add button, and enter the domain name. Click OK to close the dialog.
Run win-acme to create a Let's Encrypt SSL Certificate
Download and install win-acme from https://github.com/win-acme/win-acme.
Run wacs.exe in a command line box as administrator, and choose the N option to create the Let's Encrypt certificate. Once done, stop IIS completely.
Rerun wacs.exe, choose A: Manage renewals -> D: Show details for the renewal and copy the password of the private key.
Apply SSL Certificate in TurboFTP Server
Start TurboFTP Server service and make sure HTTPS is enabled by selecting Domain -> Connection -> HTTPS on port. Go to Server -> SSL Certs tab and click on the Import button. In the dialog, browse and choose the new certificate bundle file (the one with .PFX extension). Check off the option "Certificates and private key in a single file". Enter the copied passphrase of the SSL private key. Click OK to import the certificate.
Now go back to the Domain -> Connection tab, and select the imported SSL certificate used for the domain. Click on the Apply button and restart the domain.
The domain is now running with Let's Encrypt certificate.