In ProjectContexts, synchronizations are enabled via specific Milestones in the schedule and linking those to events outside the schedule.
Go to the relevant system where you want to place the link, press “+”.
Choose Milestone in the pop-up.

That results in (after adding a name):

Click on “attach contract event”.

The dropdown box is loaded automatically because in ProjectContexts/Contracts, the schedules of invoices are marked as events.
Save the milestone: it will be placed at the start of the project. move it vertically if desired (within the system.).
Connect the milestone to preceding tasks via a relationship (in the example below, we added a lag of 10 days).

the pop-up above displays after a double-click on the new relationship line.

The milestone information is made available to the drop-down box inside the customer contract milestones.

