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That method is not present in the stable version of the WebView2 API at this moment (1.0.1462.37).
That link shows some code that uses a ICoreWebView2_15 interface which apparently includes methods like "Print" and "ShowPrintUI" but the current stable WebView2 API doesn't have those methods :
https://learn.microsoft.com/en-us/microsoft-edge/webview2/reference/win32/icorewebview2_15?view=webview2-1.0.1462.37
I guess Microsoft might decide to include those methods in ICoreWebView2_16 in the future.
PS : I don't work for Microsoft nor Google so the last comment is pure speculation.