This can be done through the MyFXBook website by default, without Fusion Markets needing to make any changes.
This should be under Portfolio > Add Account > cTrader which should direct the client to the Spotware website to log in using their cTrader ID.
Unfortunately, as this process is outside of Fusion Market's control, if they encounter any issues setting this up then MyFXBook support would be the best contact point for resolving the issue.