How do I add my account to a second-hand Xbox that has an access key?

Outside of getting the code from the original owner, you'd have to contact support so they can give you a code to reset it. http://support.xbox.com/contact-us

No that is not the problem the person who has used my gift key is not me i have no hands on that account.

Something went wrong with microsoft's key protection and someone else got their dirty hands on my key.

That is the problem... Why cant microsoft delete a minecraft gift key on the other persons account?

Thank you for enquiring my question however my issue is that i cannot sign into an adult account on the xbox which is required to change or remove a guest key- and i cannot access the guest key because iv forgotten it which is required to add an adult
account.

To do this, there is a few steps you have to take before hand in order to add a new account to the system.

1. Simply create a new Microsoft account.

2. Once this is set up, go to "Add or Switch," and select the "+ Add New."

3. Enter in the necessary information from the newly created Microsoft account, enter in the gamertag you want, and you should be set (It'll give you a few prompts on how you wanna set things up with the system.

Your son should still be able to access his account via the Profile & System section. All he'll have to do is press "Add or Switch" to switch back to his account in case your account is still logged into the system.

Lastly, all of the games and Xbox Live Gold should be accessible on your account if you and your son both use the same Xbox. If you ever end up getting a second Xbox, this is a guide on how to set up the main console as the home console for the second device
(Guide can be found
here).

If you're trying to add your account to another Xbox you only need to login with your Microsoft account and everything syncs. If you're trying to add another account to your Xbox it'd be the same. Just login to that account on your Xbox. If you're attempting
to accomplish something else we'd need more information about what you're trying to do.