Short answer: Put the executable file in /usr/local/bin instead of applications. You should now be able to run commands like ngrok http 80.
Long answer: When you type commands like ngrok in the terminal, Macs (and other Unix OSs) look for these programs in the folders specified in your PATH. The PATH is a list of folders that’s specified by each user. To check your path, open the terminal and type: echo $PATH.
You’ll see output that looks something like: /usr/local/bin:/usr/bin:/bin. This is a : separated list of folders.
So when you type ngrok in the terminal, your Mac will look for this executable in the following folders: /usr/local/bin, /usr/bin/ and /bin.
Read this post if you are interested in learning about why you should prefer usr/local/bin over other folders.