@zer0kill It looks like you're posting from the Official App, which doesn't currently have the interface for that.
Via the web interface when you search a hashtag, there should be the little 'follow' icon.
@zer0kill I'm not exactly sure why the official app is terrible, but it is.
I don't use Android myself, so I can't speak to it, but @Texan_Reverend has a thread on his recommendations here:
I'm going to formally take the stance that the Mastodon PWA is the best app to start with on Android. iOS won't allow it to push notifications, so I can't recommend it as easily unless you don't mind that or workarounds. The PWA (Progressive Web App) is basically the mobile-formatted website in a standalone app shell. It doesn't hide options or features like many apps do - including the official one. It also makes clear when a profile isn't displaying all their posts due to federation quirks. 1/