To apply to have your bot included in Discover, have your Page admin complete the Discover submission form.
You can also navigate to the through Page settings:
Click Settings at the top of your page
Click Messenger Platform in the left column
From here, you can choose the categories where your bot appears, customize how it's described in search results, and decide which languages you want to support. This information will be used to enhance your bot description in Discover and will also help improve its discoverability.
Please note that each bot which are approved by Facebook will be enable at the bottom of your Messenger screen.