REALM Append list into linkingObjects - swift

Question

I want to use realm to append all data in one VC.
parentVC is showing Category List, in ParentVC can only append new Category
childVC is showing toDoItem List, and can only append to selectedCategory.toDoList
I created a new VC "addToDoListVC", to achieve result of "Adding new toDoItems to selected todoListCategory".
below is my code and image shown is the new addToDoListVC layout.
addToDoItemVC layout
class Category : Object {
#objc dynamic var categoryName : String = ""
let ofToDoItem = List<ToDoItem>()
}
class ToDoList : Object {
#objc dynamic var toDoItem : String = ""
var parentCategory = LinkingObjects(fromType: Category.self, property: "ofToDoItem")
}
addItemVC
let realm = try! Realm()
var categorySelection : Results<Category>?
#IBOutlet weak var categoryName: UILabel!
#IBOutlet weak var categoryNamePicker: UIPickerView!
#IBAction func addItemBtnPressed(_ sender: UIButton) {
if let currentCategory = self.categorySelection {
do {
try! realm.write
let newToDoItem = ToDoList()
**// HOW DO I ADD THE ITEM TO SELECTED CATEGORY?**
newToDoItem.toDoItem = textField.text
selectedCategory[**???**].ofToDoItem.append(newToDoItem)
**[// I can append data into selected category by "selectedCategory\[0\].ofToDoItem.append(newToDoItem)" but it is hardcoded.][1]**
}
}
}
//MARK:- PICKERVIEW METHODS
extension ParentVC : UIPickerViewDelegate, UIPickerViewDataSource {
func numberOfComponents(in pickerView: UIPickerView) -> Int {
return 1
}
func pickerView(_ pickerView: UIPickerView, numberOfRowsInComponent component: Int) -> Int {
return categorySelection?.count ?? 0
}
func pickerView(_ pickerView: UIPickerView, titleForRow row: Int, forComponent component: Int) -> String? {
return categorySelection?[row].categoryName
}
func pickerView(_ pickerView: UIPickerView, didSelectRow row: Int, inComponent component: Int) {
categoryName.text = categorySelection?[row].categoryName
}