Found this thread https://www.hackingwithswift.com/forums/swiftui/other-custom-text-input-in-picker/7067 and wanted to reply but its already archived, so here my 2cts:

I solved it by putting the TextField and Picker in an HStack and sync them so whenever the user selects a value from the picker, that overwrites the value in the TextField. But if the user changes the TextField value to a custom value, that becomes the valid value.

@State private var exchanges = ["ADD NEW", "FWB", "ABC", "DEF"]
@State private var exchange: String = ""

HStack{
  FormField(fieldName: "Exchange", fieldValue: $exchange).textCase(.uppercase)
    .keyboardType(.default)

  Picker("", selection: $exchange) {
    ForEach(exchanges, id: \.self) {
        Text($0)
    }
  }
  .pickerStyle(.automatic)
  }

Max provides an alternate solution! Nice.

Did you create a custom component named FormField? Tell us more.

  FormField(fieldName: "Exchange", fieldValue: $exchange).textCase(.uppercase)
    .keyboardType(.default)

Yes, sorry missed that one ... :)

The idea was especially for the FormFieldDouble to have one default view for Double types so they all behave consistently and if required I just have to change the formatting in a single spot:

struct FormField: View {
    var fieldName = ""
    @Binding var fieldValue: String

    var body: some View{
        TextField(fieldName, text: $fieldValue)

    }
}

struct FormFieldDouble: View {
    var fieldName = ""
    @Binding var fieldValue: Double

    var body: some View{
        TextField(
            fieldName,
            value: $fieldValue,
            format: .number
        )
    }
}