The message box can only be displayed on the machine on which the flow runs. You cannot display it on a different machine. That's not how it works. So, if your flow actually runs on all of the machines of the involved users, you can obviously let them decide via a message box. If it runs on a single centralized machine, you cannot do that. And then you need an alternative way, such as sending a message to them via email or Teams or something like that.
It would be easier to set up via cloud flows in fact, where you can send an adaptive card via Teams. Do you have a Premium license to use Power Automate cloud flows?
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