If a Hub wishes you to self-assign to shifts in their rota they will turn this feature on for their Hub.
This means you will see shifts that you are eligible to self-assign to within your Schedule. These you can click on and easily self-assign.
Doing this creates an Assignment in your Schedule and solves that shift for the Hub – no-one else can self assign to the shift once it is claimed in this way.
Here is a short video showing how to ensure you can see self-assignment shifts in your Schedule.