Where do I find the MDN - key for my MacBook Air?

Hi M_BH,

If you're referring to Activation Lock for school or business, the MDM Bypass Code is stored directly in your MDM solution. It has 15 days after Activation Lock is turned on to retrieve and save the Bypass Code. After that, the Bypass Code cannot be retrieved from the device.

For more details, please see this article: Locking Apple devices - Apple Support

Is this a corporate MacBook Air By chance?

Per Apple: "Activation Lock is a feature designed by Apple to enhance corporate data security by mandating users to enter their Apple passwords when devices such as iPhones, iPads and Macs are factory reset."