SwiftUI How to go to another view without a NavigationLink

Question

I'm trying to access another SwiftUI View() from a function that my button calls, not directly from the button. I know that if I am to use a button I can simply do this

    Button(action: {
        self.showingDetail.toggle()
    }) {
        Text("Show Detail")
    }.sheet(isPresented: $showingDetail) {
        DetailView()
    }

But, My button

  Button(action: {
                self.makeGetCall()
                    }) {
                        Text("LOGIN") .font(.largeTitle)
                                                   .fontWeight(.heavy)
                                                   .foregroundColor(Color.black)
                    }
                    .alert(isPresented: $showingAlert) {
                        Alert(title: Text("Logging Error"), message: Text("Invalid username or password."), dismissButton: .default(Text("OK.")))
                    }

Calls a function that needs to decide which view to go to..

So within makeGetCall(), how can I go to a new view?

Here's the current code for makeGetCall()

func makeGetCall() {
         // Set up the URL request
         let todoEndpoint: String = "https://balancingpawsdogtraining.com/api/user/generate_auth_cookie/?username=\(username)&password=\(password)"
         guard let url = URL(string: todoEndpoint) else {
           print("Error: cannot create URL")
           return
         }
         let urlRequest = URLRequest(url: url)

         // set up the session
         let config = URLSessionConfiguration.default
         let session = URLSession(configuration: config)

         // make the request
         let task = session.dataTask(with: urlRequest) {
           (data, response, error) in
           // check for any errors
           guard error == nil else {
             print("error calling GET on /todos/1")
             print(error!)
             return
           }
           // make sure we got data
           guard let responseData = data else {
             print("Error: did not receive data")
             return
           }
           // parse the result as JSON, since that's what the API provides
           do {
             guard let todo = try JSONSerialization.jsonObject(with: responseData, options: [])
               as? [String: Any] else {
                 print("error trying to convert data to JSON")
                 return
             }
             // now we have the todo
             // let's just print it to prove we can access it
             print("The todo is: " + todo.description)

             // the todo object is a dictionary
             // so we just access the title using the "title" key
             // so check for a title and print it if we have one
             if let status = todo["status"] as? String
             {
                    if status == "error"
                    {
                        self.showingAlert = true
                        return
                    }
                    else
                    {

                        AppVariables.cookie = todo["cookie"] as! String
                        self.showLinkTarget = true
                                   print("The title is \(todo["cookie"] as! String)")

                        MainScreenView()

                    }

            }

           } catch  {
             print("error trying to convert data to JSON")
             return
           }
         }
         task.resume()
       }

I tried

   AppVariables.cookie = todo["cookie"] as! String
                        self.showLinkTarget = true
                                   print("The title is \(todo["cookie"] as! String)")

                        MainScreenView()

Calling just MainScreenView() --- this is the name of the View I'd like to go to in this situation, but that does nothing.

So how Can I turn that MainScreenView() into actually navigating to MainScreenView()

---- EDIT ----

I figured out I can just do this

}.sheet(isPresented: $showingDetail) {
                MainScreenView()
            }

and change the showingDetail in my called function.

However, this "Overlays" my current view with the new one, how do I completely switch to it?