In Outlook desktop app, click on File, then Manage Add-ins.

That will open up your Office 365 account in the web version, with the Add-ins pop-up. You can then search for FindTime in the search bar:

Then just click to add it to your Outlook:

Once you’ve done that, when you open to send a new message, the New Meeting Poll button should show up in the ribbon.

For more details on the polls, follow these instructions from Microsoft.