Since the default channel is designed to be a draft channel and it's private, you should create a new channel on imean builder, and then move all the tasks you want to include in the challenge to that channel.
Move the Tasks to the Created Channel
Create a Challenge bonded with that Channel
Then you can create a challenge on WebCanvas platform and connect it with the channel in step 1.
Create a Challenge and Connect it with the Channel
Export the Data and Run the Evaluation
Check out our GitHub repository for the complete user story on running online evaluation after you set up a challenge. https://github.com/iMeanAI/WebCanvas