You are sign in here . . . with your Apple ID & Password.
Your Apple ID is the personal account you use with all Apple products and services. It includes the email address and password you use to sign in, as well as the contact, payment, and security details that you use with Apple.
You can’t merge multiple Apple ID accounts.
is their APPLE ID thereby DEFAULT
Who is 'their' Apple ID?
See -> If you can't connect or sign in to iCloud
If you can't connect or sign in to iCloud - Apple Support
See also -> How to remove Activation Lock
How to remove Activation Lock – Apple Support (UK)
You can contact Apple support for help, -> Get Support (apple.com)
You have to show proof of ownership of your product, provide all documentation showing your purchase of the device.
If you do not have additional proof of ownership, contact the reseller. They may be able to provide the original purchase document.
Once Apple receive your proof of ownership, Apple will further review your request and provide an update.