How to configure iOS 12 sharesheet to add WhatsApp Messaging?
If the developer didn’t program the hooks to receive a forward, you cannot accomplish this. Contact the developer or choose another app that has that feature. 

If the developer enabled this, the mechanics are easy to curate your list of share destinations:


Tap share, swipe to the right end and tap “more” for shifting the order of apps, and also enabling/disabling them.
Scroll towards the bottom and you'll see all apps that are allowed to use sharesheet. 




It’s possible a third things could happen, which would be corruption of the data store that holds your customizations. I would back up the device and then erase the device and install only WhatsApp to validate. Use the "three bar" icon to shift it in the list. If it doesn't appear, then seek assistance from Apple or hope restoring your full backup allows the addition.