The webapp manifest of a PWA has a orientation property and can be set in the manifest-overrides.manifest file. This directive is however not supported on safari ios.
https://docs.mendix.com/refguide/mobile/introduction-to-mobile-technologies/progressive-web-app/#51-web-app-manifest
https://developer.mozilla.org/en-US/docs/Web/Manifest/orientation
An alternative would be to use css to display a warning that portrait mode is not supported and the user should turn his screen.