To synchronize your iCloud calendar in Bitrix24, you need to get the application password from Apple. This is a standard operation for connecting to third-party applications.
How to get the application password
Log in to your Apple account – appleid.apple.com.
In the Sign-In and Security section, click on the App-Specific Passwords block.
Click the Generate an app-specific password.
Specify the service name and click Create.
Copy the application password you created.
Enter it in the Application password field when you connect the sync in Bitrix24.