I just tested this and it worked for me. After booking here are the steps to see your reservation on Sixt.com:
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1. Go to https://www.sixt.com
2. Click on the Manage Bookings link at the top of the page
3. Enter the same email address you used when you made the booking on AutoSlash
4. Enter your first and last name and choose a password
5. Once you've registered, you will receive an email from Sixt to confirm the account. You must complete this step by confirming your email address in order to see your booking.
6. Once you've confirmed your email address, you can click on the Manage Bookings link at the top of the page again and you should see your reservation there.

Sixt should also send an online check-in email as well, but that may not be sent until closer to the pickup date.

Hope this helps!