Short answer
Go to Public Interface option from the Manage Index menu, select Create Interface, give it a name, then configure and save.
Full explanation
Make sure the correct Index is selected at the top of the app.
Open the Manage Index menu
Click Public Interface in the left menu.
Select Create Interface and enter the name you want to use for the Public Index.
On the setup screen, use Basic Settings to enable the interface and name it.
Use User Interaction to set the welcome message and any default prompts.
Use Integration to review the live preview and prepare the embed for your site.
Select Save Changes.
Tips
Each interface is tied to the Index that was active when you created it. If you need a bot for another Index, switch Index first and create a new interface or duplicate then re-create in the other Index.