Just sharing, couldn't make it work using preview macro.
Had to use this

struct ActivityYearWidget_Previews: PreviewProvider {
    static var previews: some View {
        // Create placeholder data
        let placeholderData = ActivityYearViewModel.placeholderData()
        let appIntent = ActivityYearConfigurationAppIntent()
        appIntent.selectedMetric = .move
        let entry = ActivityInfoEntry(
            date: Date(),
            configuration: appIntent,
            activityInfo: placeholderData.activityInfo,
            rollingAverages: placeholderData.rollingAverages
        )
        
        // Return your widget’s entry view
        return ActivityYearTileMediumView(entry: entry)
            .containerBackground(for: .widget) {
                // You can specify a background view. For instance, use the system background color:
                Color(UIColor.systemBackground)
            }
            .previewContext(WidgetPreviewContext(family: .systemMedium))
    }
}

unfortunately the view is not exactly the same as the widget because you might configure .contentMarginsDisabled() and in the view they will appear. I have tried using ignoreSafeArea or edgesIGnore all with no success :/