How to add the UISegmentedControl in UINavigationBar?

I think you are looking for this.

let searchVC = self.storyboard?.instantiateViewController(withIdentifier:"idofcontroller")
let searchController = UISearchController(searchResultsController: searchVC)
searchController.searchResultsUpdater = self
searchController.obscuresBackgroundDuringPresentation = false
searchController.searchBar.placeholder = "Search"
navigationItem.searchController = searchController
definesPresentationContext = true

searchController.searchBar.scopeButtonTitles = ["News", "Photos", "Videos"]
searchController.searchBar.delegate = self

This is how I do it in Objective C (sorry, I haven't learned Swift yet):

- (void)viewDidLoad
{
    [super viewDidLoad];

    self.viewControllers = [self segmentViewControllers];

    self.segmentedControl = [[UISegmentedControl alloc] initWithItems: [self.segmentedControl addTarget: self
                          action: @selector(segmentClicked:)
                forControlEvents: UIControlEventValueChanged];

    CGFloat topOffset = self.navigationController.navigationBar.frame.size.height + [UIApplication sharedApplication].statusBarFrame.size.height;

    self.toolbar = [[UIToolbar alloc] initWithFrame: CGRectMake(0, topOffset, self.navigationController.navigationBar.frame.size.width, kDefaultViewHeight)];
    self.toolbar.delegate = self;

    self.navigationController.navigationBar.backgroundColor = [UIColor whiteColor];
    self.toolbar.backgroundColor = [UIColor whiteColor];
    self.toolbar.clipsToBounds = YES;

    UIBarButtonItem *segmentedControlItem = [[UIBarButtonItem alloc] initWithCustomView: self.segmentedControl];
    UIBarButtonItem *flexibleItem = [[UIBarButtonItem alloc] initWithBarButtonSystemItem: UIBarButtonSystemItemFlexibleSpace
                                                                              target: nil
                                                                              action: nil];

    [self.toolbar setItems: @[flexibleItem, segmentedControlItem, flexibleItem] animated: YES];

    [self.navigationController.view addSubview: self.toolbar];

    self.segmentedControl.selectedSegmentIndex = 0;
}

And in UITableViewController (one of the segmentViewControllers):

  self.tableView.contentInset = UIEdgeInsetsMake(topOffset, 0, 0, 0);

    CGRect currentFrame = self.tableView.frame;
    [self.tableView setFrame: CGRectMake(currentFrame.origin.x,
                                     currentFrame.origin.y,
                                     currentFrame.size.width,
                                     currentFrame.size.height + [UIApplication sharedApplication].statusBarFrame.size.height)];

To put it in the navigationBar has a right left and title view for such things... accessed using....

self.navigationItem.titleView = mySegmentedControl

for future reference....

self.navigationItem.rightBarButtonItem
self.navigationItem.leftBarButtonItems // for adding an Array of items

To add it below the navigation view but have it static.. add it to the viewController.view... Are you using a UITableViewController? If so maybe switch to a UIViewController and add a tableView then your toolbarview as subviews to that self.view.

To add segment control in UINavigationBar in Swift 5

    let segment: UISegmentedControl = UISegmentedControl(items: ["First", "Second"])
    segment.sizeToFit()
    if #available(iOS 13.0, *) {
        segment.selectedSegmentTintColor = UIColor.red
    } else {
       segment.tintColor = UIColor.red
    }
    segment.selectedSegmentIndex = 0
    segment.setTitleTextAttributes([NSAttributedString.Key.font : UIFont(name: "ProximaNova-Light", size: 15)!], for: .normal)
    self.navigationItem.titleView = segment