There is no external approval (running a wf) like in vRA7.x.
I did something similar earlier for deployment request. The goal was to check ServiceNow approval status and continue if approved there. I created an API user and a background WF approved the request via vRA REST API. This could be adapted but requires custom code of API calls.
What is easier is to call an EBS WF and fail if your conditions are not met. If it is a simple, quick decision, I would not involve approval policies in the solution.