How to log in icloud iPad 1?

Due to the age of your IOS, I suggest you turn 2 Factor Authentication off for your Apple ID. You can do that at the link below under Security.

Apple ID - Manage

Two-factor authentication for Apple ID - Apple Support

You need at least iOS 9 to properly login. You can try entering the verification code after your password but I've only see that work on Macs.

By that I mean enter your password with the 6 digit code at the end in the password field.

Then your iCloud will be unavailable on your iPad. Sorry.