If you are using PyInstaller>=3.6, it should work out of the box as there is hook that takes care of the bundling of necessary dlls. Therefore, this version of PyInstaller is the recommended one.
Should you need to use prior versions of PyInstaller (<=3.5), you will need to bundle the dlls yourself. Either WebBrowserInterop.x86.dll (opens new window) or WebBrowserInterop.x64.dll (opens new window) depending on whether you build against 32-bit or 64-bit Python.
The DLLs bundled with pywebview and are located in the