I managed to find out how Apple posts the request, which can be seen and captured with:

@Post('/:version/devices/:deviceLibraryIdentifier/registrations/:passTypeIdentifier/:serialNumber')
    async logPost(
        @Headers() headers: any,
        @Param('deviceLibraryIdentifier') deviceLibraryIdentifier: string,
        @Param('passTypeIdentifier') passTypeIdentifier: string,
        @Param('serialNumber') serialNumber: string,
        @Query('passesUpdatedSince') passesUpdatedSince: string,
      ): Promise<any> {
        this.logger.log('Received POST from Apple');
        this.logger.debug(`Headers: ${JSON.stringify(headers)}`);
        this.logger.debug(`Params: ${deviceLibraryIdentifier}, ${passTypeIdentifier}, ${serialNumber}`);
        this.logger.debug(`passesUpdatedSince: ${passesUpdatedSince}`);
        const date = new Date();
        return { lastUpdated: date.toISOString() };
      }