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Hi, you're welcome.

It's the private active key of the account you'll be using as a witness. So it is one of the infamous four Hive key pairs :)

You put the public active key in the form here (https://tribaldex.com/witnesses) by clicking on the Manage Witness button.

There you can also enable / disable your witness node.

The same can be done via script by calling:

 node witness_action.js register

or

 node witness_action.js unregister

Good luck.

Also, check @blervin's latest post.Hive Engine Script Updates.

Good luck!