$data = (object) array(
    'opportunity' => (object) array(
        'description' => 'Scope and design web site shopping cart',
        'party'       => array( 581 ),
        'value'       => array(
            'amount'   => 500,
            'currency' => 'USD'
        )
    )
);

$url            = 'https://api.capsulecrm.com/api/v2/parties/opportunities';
$params         = wp_fusion()->crm->get_params();
$params['body'] = json_encode( $data );

$response = wp_remote_post( $url, $params );