How do I get it back into my account
Do you mean your account on your old MacBook Pro?
1.
It depends on what you did with your 'that Apple ID'. If you have deleted it, it is no longer valid, it simply no longer exists and it no longer works.
To understand about a deleted Apple ID, see the Apple articles below:
How to delete your Apple ID account -> How to delete your Apple ID account – Apple Support (UK)
Understand and control the personal information that you store with Apple
Understand and control the personal information that you store with Apple - Apple Support
2.
If you stil have / know that Apple ID, check your Apple ID via your email box, see this Apple article: If you forgot your Apple ID
If you forgot your Apple ID email address or phone number - Apple Support
OR see -> Having trouble signing in? -> https://iforgot.apple.com/password/verify/appleid
Try this -> If you can't connect or sign in to iCloud -> If you can't connect or sign in to iCloud - Apple Support
See also -> Contact Apple for help with Apple ID account security
Contact Apple for help with Apple ID account security - Apple Support
3.
Do you use a 'new' Apple ID on your MacBook Air?
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.