How can I add a bottom line on TextField (SwiftUI)

Divider

A visual element that can be used to separate other content.

You can set color and height

Divider()
 .frame(height: 1)
 .padding(.horizontal, 30)
 .background(Color.red)

struct LoginField: View {

    @State private var email: String = ""
    @State private var password: String = ""

    var body: some View {
        VStack {
            TextField("Email", text: $email)
                .padding(.horizontal, 30).padding(.top, 20)
            Divider()
                .padding(.horizontal, 30)
            TextField("Password", text: $password)
                .padding(.horizontal, 30).padding(.top, 20)
            Divider()
                .padding(.horizontal, 30)
            Spacer()
        }
    }
}