API: assign opportunity owner

…in the meantime, there is my workaround using a raw API call:

            $opportunity = $isoft->opportunities()->create([
                'opportunity_title' => $data['opportunityTitle'],
                'opportunity_notes' => $data['opportunityNotes'],
                'contact' => [
                    'id'           => $data['contactId'],
                    'email'        => $data['email'],
                    'phone_number' => $data['phone']
                ],
                'stage' => [
                    'id' => 123
                ]
            ]);

            // $http here refers to GuzzlePHP, but any http client will work
            $http->request(
                'PATCH',
                "https://api.infusionsoft.com/crm/rest/v1/opportunities/{$opportunity->id}",
                [
                    'headers' => [
                        'Authorization' => "Bearer {$isoftToken}"
                    ],
                    'json' => ['user' => [
                        'id' => $opportunityOwnerId
                    ]]
                ]
            );