Goto Settings > Maintenance windows > Maintenance windows for monitoring, alerting, and availability > Create Maintenance window
You can do this in 2 ways,
1. Use the Entity filter and add the service "requests to unmonitored hosts" to the maintenance windows.
2. You can also use the tag associated with that service and add that tag to set up a maintenance window for the service and the requests under that.
That isn't possible unfortunately.
What you can do, is split the "Requests to unmonitored hosts" service into multiple services and than apply a maintenance window to one of those.
Not sure how to do it using the UI, but there is an API for it, see: New Dynatrace API enhances automatic service detection | Dynatrace news