Now that you have set all but one of the environmental values, you can request the last value, the oauth_access_token. In the oauth folder on the left hand side, you can find the Request an Access Token POST.
Simply select this POST function and hit the send button. This will create a new access token. This token is valid for 24 hours. After it expires, you can easily create a new one by repeating the above.
Now that you have created an access token, you can copy it and use it in your environmental variables by pasting it in the value box of the oauth_access_token key.
After pressing the update button, you are all set to work with our API.