External Tool (custom) integration for environment booking during Release

[snowdev8]

We have a use case where the process would be - as soon as the release enters the testing phase, a booking request needs to be sent to the external environment for 'specific dates'. 
How would it be possible to denote specific start and end dates as part of the booking process?
Would it be a Catalog Item with an external API integration?
In that case, is there a way to clone OOB policies to force the user to ensure that the request has been submitted?

[Siva Sankar Ala]

We don't have such OOTB policies to enforce that.
But to meet your requirement I suggest you to add a task which is the first task in Testing Phase.
As soon as the Testing Phase started that task get's opened and you can trigger some Business rule to do the necessary steps you required. Once the request is completed you can mark the task as completed as part of the Business rule.
You can leverage All Planned Tasks are Completed to for checking