[CentOS] systemd | Requires statement with an instantiated service

Maybe the work day is already to long. I can not find a solution for following requirement.

How to apply a "Requires" with an instantiated service.

Example:

a@.service b.service

a@.service is started as a@host1.service and b.service must be started after a@host1.service but the unit will be differently parameterized (depended of the region). So I want to generalize the requires statement.

My dropin file in ./b.service.d/dep.conf looks like

[Unit] Requires="a@*.service"

This just produces following error: 'Failed to add dependency on "a@*.service", ignoring: Invalid argument'

I use also a Before=b.service statement for a@.service but that is not enough.

Any hints?

-- Thanks, Leon