To securely sign in to any third-party app that requires iCloud login, follow these easy steps. (generate and use app-specific password for iCloud with your desired application).
Step 1:- Sign in to your Apple ID account at "appleid.apple.com".
Step 2:- From the “Sign In and Security” screen, choose App-Specific Passwords.

Step 3:- Next, press the "Generate an app-specific password" button.

Step 4:- Enter the app name (add any name) and click on the "Create" button.

Step 5:- Get new app-specific password for iCloud account login, copy it, and click on "Done".

Step 1:- Open RegZion iCloud Backup Software (or the third-party app you're using).
Step 2:- When inputting iCloud login credentials, paste the app password you generated earlier in the password field.
Step 3:- Complete the setup according to the instructions in the app.