If you were fortunate to use the Outlook add-in while it was part of the Outlook client, the functionality went live to the O365 Outlook.com site.
https://outlook.office365.com/owa/
Create a meeting and Poll for other times
Login to Office.com with your credentials
Click on Outlook
Click on Calendar button
Click on dropdown, New
Fill out meeting invite, and don’t forget to propose meeting times
Hit send and wait for the attendees to vote
High level overview
Post your comments to have this added to the Outlook App
UserVoice Forum for Outlook https://outlook.uservoice.com/forums/313228–outlook-on-the-web-office-365/suggestions/31651732-meeting-poll-feature