Hi KF,
Unfortunately, Roadmap Item 558928 is currently listed as In Development. The feature is still under development and is not yet available. Therefore, there is no option for early access or manual enablement. Once it reaches General Availability, it will be rolled out automatically to eligible tenants.

If this functionality is business-critical, I recommend monitoring the Microsoft 365 Roadmap entry and your tenant's Message Center for any updates regarding preview availability or rollout timing. In the meantime, you may want to consider using a standard channel with a Planner tab or managing the plan directly in Planner until support for shared channels becomes available.
I hope this provides enough clarity and helps you stay inform with your current situation.
Thank you for your patience and your understanding. If you have any questions or need further assistance, please feel free to share them in the comments on this post so I can continue to support you.
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