To obtain an OAuthClientId, OAuthClientSecret, and CallbackURL, you first need to create an app linked to your BigCommerce account.

To create an app linked to your BigCommerce account:

1. Log in to the The BigCommerce developer portal.
2. In the left-hand sidebar, click Apps > Overview to open the Overview page, then click + Create App.
3. Assign a name to your app by entering the name into the Give your app a name field, and then clicking Generate Credentials.
4. Note the value in the Client ID and Client Secret fields for later use. Note that the Client Secret field is hidden, so you must either click the copy button to copy it or click the eye icon to show the value. Then, click Finish. A new page containing details of your app loads.
5. Under Callback URLs, set the Auth field to the URL you would like to be redirected to after completing OAuth authentication, such as http://localhost:33333.
6. Under App users, enable the Enable multiple users if you want your app to be accessible by multiple users, or disable it if you only want the store owner to access the app.
7. On the Scopes tab, under OAuth Scopes, enable the scopes corresponding to the permissions you would like your application to have, then click Save.