Welcome to the community, @Santiago_Milito! :grinning_face_with_big_eyes: You need to read that metadata API page a little more closely. The second line reads (with emphasis added by me):
If you want to develop an integration using the Metadata API, you must register here for access and to receive a client secret.
Once you receive that secret from Airtable, you can include the X-Airtable-Client-Secret header with your requests.