Moodle Use Webservice for store data

I’m currently trying to write data from the app into the database. I’m using the web service for this.

However, I get the following error message and the documentation is, in my opinion, very sparse.

core_externalexternal_multiple_structure::__ construct(): Argument #1
($Content) must be of type core_externalexternal_description, array
given, called in

The relevant line is the following:

'eintraege' => new external_multiple_structure([

The complete function is:

public static function execute_returns(): external_single_structure {
        return new external_single_structure([
            'eintraege' => new external_multiple_structure([
                    'fach' => new external_value(PARAM_INT, 'FachID'),
                    'datum' => new external_value(PARAM_RAW, 'Datum des Eintrags'),
                    'ustdbis' => new external_value(PARAM_INT, 'USTD bis'),
                    'ustdvon' => new external_value(PARAM_INT, 'USTD von'),
                    'eintrag' => new external_value(PARAM_TEXT, 'Eintrag'),
            ])
        ]);
    }

I would be very grateful for any help and thank you very much for that!

I tried the following variations:
`A parameter can be described as:

a list => external_multiple_structure
an object => external_single_structure
a primary type => external_value`

As described here:
https://moodledev.io/docs/4.5/apis/subsystems/external/writing-a-service