I have found a few redeem codes for Todoist Premium. You should be able to use them even if you are already on Premium. I was able to extend my account for 17 months of Premium.
Visit - https://todoist.com/redeem
And type one of following Serial Code, click on UPGRADE, and repeat the process with another one:
Productivityist
TDSANEBOX
skillshare2018
FreelancerClub
TodoistFreedom
Another possibility to get 2 months premium:
Import your tasks from Wunderlist through this link - https://en.todoist.com/import/wunderlist. You will receive an email with a code for 2 months. The code is always the same (6K94QXHTQ6) and maybe could be used without Wunderlist import. I did the import first so I couldn’t test this possibility.
Another possibility to get premium:
Enable Karma and then once you reach the professional level you should get free 3 months of premium for you and your friend. You can swap your codes here - https://www.reddit.com/r/todoist/comments/78fzjs/code_swap_thread to get another extension. The same process can be done after reaching the expert level. But I haven't tested this possibility.
