When you buy a challenge for the cTrader platform and you do not know your cTrader ID password, please note that you have received 2 emails to your email inbox directly from the cTrader platform provider. First email with your login information for the challenge (Trading Account Opened) and second email about setting password for your cTrader ID ([ACTION REQUIRED] Set Password for cTrader ID Account).

Click on option "Set password" and successfully finish this step. Then you can open your cTrader platform and log in with your cTrader ID email address and password you just set. You will find all your purchased accounts under your cTrader ID in the platform.

You can find a lot of useful information about cTrader here - https://help.ctrader.com/ctrader-id/.