Consider carefully reading How should I ask a question in the Forum if I want to receive useful answers?
Especially the "what you want to achieve" part and provide that information in the question.

As far as I can tell, there is NO extension point for server startup and I don't see why that would be required. If you want to run an extension, then run it in the operations provided.

As far as I can tell, all the extensions are equally deployed by the Equinox/Eclipse OSGI mechanisms and started sometimes during the server startup. As far as I can tell, you should be able to do some stuff in the initial start phase.

Otherwise it is possible to have scheduled execution as described in https://rsjazz.wordpress.com/2015/10/16/due-date-notifier-an-asynchronous-task-example/