Thank you for the information. Under every cartoon challenge page bottom of the submitted content, you will see a submission form where you can submit your entry. Example here https://www.toonsmag.com/social-distancing-cartoon-challenge/
Here is a tutorial about submission, it might be helpful https://toonsmag.uservoice.com/knowledgebase/articles/1915126-how-to-add-an-entry-into-open-list
Please let me know if you still face the issue, I am always here to assist you.