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You would need to first go to the printer's driver settings... found in
Control Panel - Devices and Printers - right-click on the icon for the Epson XP810 printer - choose Properties, and then, you are in the driver settings.
Go into those settings, and find the Paper Size setting. Choose the right type of Envelope... or if there is not choice for envelopes of the right size, choose Custom, and set the dimensions of width and height for the envelopes you are using.
Click on OK until the Properties close, to save the changed setting.
Then send the print job after loading envelopes according to your printer's instructions in the manual.