If you are stored in the system as an employee, you generally have the authorization to apply for shifts.
This is how to apply for a shift as an employee:
Log in with your employee profile.
Click on the calendar.
Now you can apply for shifts by just clicking on the respective shift.
If the shift turns yellow, you have successfully applied for this shift.
If you have received the "Self-assignment" authorization from an Admin, from now on you can independently assign yourself to shifts that the Admin created.