You figured it out in the time it took me to draft this but for future reference for anyone else…
How to make a poll
In a text field, click on the cog wheel
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Click Build Poll
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Enter the parameters for your poll. Please note, each poll has different parameters
Click the Insert Poll button
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The poll will now be set up in the text field.
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Click Message to post the poll
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Once you post a poll, you can only edit it for 5 minutes. Any votes cast during those 5 minutes will be erased.