Short version: you don't.
If the email contains a 'https://' link, then the OS is going to open that with your default browser.
The only way to get the link to open in Facebook would be to reconfigure 'https' URLs to go to FaceBook instead, but that's not going to work for 99% of the links that you click, that aren't on Facebook.
The two options here are: 1) Facebook need to provide a different URL scheme (other than https) that tells the OS to open in Facebook rather than the app, or b) you go through the web interface and click the button there to transfer to the app.
Neither approach is ideal.
In order for links to point directly to the app, the email would have to know which device it's on - if it's on iOS then open in the app (assuming the app is installed). Otherwise the link would just fail on non-IOS devices (e.g. MacOS or Windows devices) that don't have a dedicated Facebook app.