How to set a default Tab in SwiftUIs TabView?

Define a State with the default value and bind it to TabView:

@State var selection = 1
,,,

    TabView(selection: $selection) 

,,,

Then assign a tag to each tabItem:

,,,
    .tabItem {
        Image(systemName: "bolt.horizontal.fill")
        Text("Trend")
    }.tag(1)
,,,

Now you can use them together, selection === tag

@State private var selection = 3

TabView(selection:$selection) {
     QuestionView()
          .tabItem {
              Image(systemName: "questionmark")
              Text("Questions")
          }
          .tag(1)
     DataView()
          .tabItem {
              Image(systemName: "chart.bar.fill")
              Text("Data")
          }
          .tag(2)
     Text("Empty Tab right now")
          .tabItem {
              Image(systemName: "bolt.horizontal.fill")
              Text("Trend")
          }
          .tag(3)
}

In this case I set default selected the third Tab. Change selection to the desired Tab.