That is one feature that is missing from the app. The android forum app is a special skinned Tapatalk app for this forum. Tapatalk also does not have the thank feature built in.
What I end up doing is opening the post in a browser window (tap the post, more, open in browser) and then thanking from there. Not the best, but better than not thanking.
Unfortunately since it's an app that is coded by Tapatalk, we don't have the ability to make changes. We can request one, but I don't think the changes of getting them to do it are very high. Very sorry.