Your payload is wrong for this request.
(PHP)
$payload = array(
'exp' => time() + 60,
'user_id' => '15185913',
'role' => 'external',
);
Is the correct payload.
- exp - Time in seconds (and add a bit)
- user_id - the UserID of the person who OWNS the extension, as a string
- role - external is the only value