Understanding webhooks

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I think I managed to fix it

add_action('rest_api_init', function(){
    register_rest_route('endpoint', '/sportivity', [
        'method' => 'POST',
        'callback' => 'sportivity_endpoint',
    ]);
});

function sportivity_endpoint($request){
    $json = $request->get_json_params();
    $json['test'] = 'Jan';
    return $json;
}

When I go to url/wp-json/endpoint/sportivity it returns the json now.