There are fixed dates to sign up for your master thesis. Each semester there are three dates, you can find them here:
There will be a waiting list on the door of the exam office where you have to put your name in. These dates are the only ones when you can sign up for your thesis.
Current dates: 4. September 2018, 6. November 2018, 4. December 2018 und 5. March 2019.
If you can't show up in person an authorized person can do it for you.