Have you tried Automation rule → Ticket update, agent changed from any to any agent ? 

Suggested the same earlier but customer wants to do the action explicitly using API/webhoook on powerautomate

So don’t want the built in automation system. Then I guess a server less FD app that dose the same thing. 

Does this means custom app is the only go ?

Custom app yes as far as I understand the what they wan’t. But it will be about the same as the build in automation with the app trigger on agent change and it makes a API request to the Power Automate. 

You could do a GET on all tickets from a external app like Power Automate but there are a limit on how many requests you can do per minute so they need to build a cue system for let say 70% of max requests per minute.  But then you need to store all tickets and match them to the new status. It’s not a good way to do it. 

Understood. thanks a ton Daniel :)