faire.com

Create a developer account in the Faire Developer Portal, then create/register your application there to obtain an applicationId and applicationSecret. The API docs state these are used together as Base64-encoded applicationId:applicationSecret in the X-FAIRE-APP-CREDENTIALS header. For custom integrations, the help center says you build your app first, then in your Faire portal go to Settings → Integrations → Have an unpublished integration? and enter your app token to continue generating access for your brand.

For a single-brand/custom integration, generate the API key (access token) in your Faire portal: go to Integrations from Settings, then use Generate API key for a partner integration or Have an unpublished integration? for a custom integration as described in How do I get an API key?. For multi-brand partner integrations, use the OAuth Authorization Code flow documented in the Faire External API docs: register an app in the Developer Portal, send the user to https://faire.com/oauth2/authorize?..., exchange the authorization code for an access token, then use that token in X-FAIRE-OAUTH-ACCESS-TOKEN.