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Start by selecting the desired Team from the list:
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Select the Add button from the top once the Team is open. It looks like a plus symbol (+):
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Select OneNote from the list of apps to choose from:
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From here you can choose to add a new notebook or select the default notebook. The recommended choice is the default notebook:
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This will add the notebook as a tab to select at the top of the Team when it is opened:
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