Authoring from chat
Unfortunately, at this time, we do not support scheduling a poll to post in the future from the chat. To do so, you will need to author a poll from the pinned Polly tab or the Polly message extension.
Authoring from Polly tab / Polly message extension
At the very top of the authoring UI from the tab, the poll is scheduled to send immediately by default. But if you wish, you could choose to schedule the poll to send in the future.
From the Poll Builder (via message extension), you'll need to click on the Advanced Options button to bring up the ability to schedule a poll.
You can customize the date to send the poll, as well as the time. When picking a date, you can either enter in the date yourself, or choose from the date-picker (the drop-down) as shown below.
Once you have created the poll, it will send at your desired date/time. Until it sends, it will live in the Poll Results tab, if you need to delete the poll preemptively.