Secure print will be required starting in October 2022. In order to use the STAR WebPRNT direct print functionality, secure print must be enabled after this date.
Secure print is only available when using WebPRNT. WebPRNT is the only direct print option available to send receipts to your Star receipt printer without any dialogs displaying.
In order to use your printer in secure print mode, you will have to follow these steps in order for your device to "trust" your printer. You will log in to your printer over the network, create a certificate, then import that certificate into your device.
First, make sure your printer is connected to your network and you know the IP address of the printer. To get the IP address, turn off the printer, then hold down the paper feed button while you press the power button once to turn it back on. The printer will print out the IP address of your printer at the very end.
Next, make sure secure print mode is enabled. Open the POS Register (Point of Sale >POS Register), click on Printer Settings (1), make sure Star WebPRNT is selected, then enable the Secure Print toggle (3) and click Save.

Finally, follow the instructions in the appropriate sections below depending on what device you are using (Windows, Mac, iOS, etc).
Windows
Enter the IP address of the printer in the URL bar of your browser and click enter. This will take you to the printer admin page. Click SSL/TLS on the menu bar and log in. The default username/password is root/admin.

Then click Create self-signed certificate.

Click Download.

Next click on the downloaded file in in your browser downloads, or go to the location the file was downloaded and double click the file to open it.

A security warning dialog will show. Click Open.

The certificate will open. Click Install Certificate.

Click Next to start the certificate import wizard.

Select Place all certificates in the following store and click Browse

Select Trusted Root Certification Authorities and click OK.

Click Next

Click Finish to finish the import.

Click Yes on the security warning that displays.

The certificate is now imported successfully!

Finally, enter the IP address of the printer in your browser once more and hit enter. If the page displays and unsafe warning, click the 'Proceed anyways' option to go to the printer settings page. This will make sure the browser is aware the printer is safe after the certificate import.
Secure print is now set up and you should be able to print receipts directly!
Mac/iOS/Android
For all other devices, follow the instructions above to download the certificate from the printer, then follow the instructions at the link below to import the certificate to your device:
https://www.star-m.jp/products/s_print/sdk/webprnt/manual/en/_sampleProgram.htm
Troubleshooting
If you are still getting a print error, follow the steps in the troubleshooting guide.