Firstly you need to have a Forms licence. Check by logging into to Office 365, if you have Forms the icon will be displayed in all apps.
If you don't already have a Forms licence on your Office 365 account, raise a Service Request from IT helpdesk, ask for Forms to be added to your licence with reason on what you intend to do with Polls (reasoning required for GDPR, data protection).
Once you have Forms, you are good to go, follow these instructions: Poll attendees during a Teams meeting (microsoft.com)