Hello NOiSEA,
Thank you for posting in the Apple Support Communities. We'd like to help get you pointed in the right direction.
As it turns out, pressing and holding the TV button from Siri Remote will open Control Center as described here:
Use tvOS Control Center on Apple TV
However, if you're wanting to access Control Center from a third-party device, we suggest submitting feedback to Apple and/or contacting the Harmony Hub manufacturer for options with their device.
Product Feedback
Harmony Support
We hope this information is useful. Kind regards.