Creating a find dialog in C# winforms rtxtbox

So for this assignment in class I'm making a richtextbox editor in C# winforms, in this assignment I have to include a find function, but I can't seem to get the hang of it

Here's my code up until now:

            Form findDialog = new Form();
            findDialog.Width = 500;
            findDialog.Height = 142;
            findDialog.Text = "Zoeken";
            Label textLabel = new Label() { Left = 10, Top = 20, Text = "Zoek naar:", Width = 100 };
            TextBox inputBox = new TextBox() { Left = 150, Top = 20, Width = 300};
            Button search = new Button() { Text = "Zoek", Left = 350, Width = 100, Top = 70 };
            search.Click += (object sender, EventArgs e) => { findDialog.Close(); };
            findDialog.Controls.Add(search);
            findDialog.Controls.Add(textLabel);
            findDialog.Controls.Add(inputBox);
            findDialog.ShowDialog();
            return (string)inputBox.Text;

And I'm calling the function by using

string searchValue = SearchDialog();

Thanks in advance