Create channels in Microsoft Stream
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In the Stream navigation bar, go to Create > Create a channel.

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In the Create Channel page, give a unique name and description for your channel. Channel names are limited to 30 characters. Channel descriptions are limited to 2,000 characters.

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In the Channel access field select if you want your channel to be a companywide channel or a group channel. If you select group channel enter the group you want the channel to be contained in.

Note
You can't change the channel type after the channel is created.
Add a Custom channel image to make your channel look unique.
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Press Create.
Get back to your channels
After the channel is created, you can get back to your channels under My content > My channels.

Source: Microsoft