From within Airtable, click the profile icon in the top right and select Account. Or click here.
Scroll to the API section. If this is your first time using the API, you'll need to create an API key. Once created, click the box and copy the API key. You'll need that in a minute.
Note: Census's permissions will be the same as this Airtable user. If you think this Airtable user's permissions may change or the account is removed, you may want to create a special Airtable account just for Census to use.