Create Hyperlink in TextBlock via Binding

To do what you want you will have to use Inlines property of your TextBlock, but as it's not a DependencyProperty, it cannot be a target of binding. We will have to extend your TextBlock class, but as it's sealed we will have to use other class.

Lets define static class, which will add apropriate Inline - Hyperlink or Run, depending on Regex match. It can look for example like this:

public static class TextBlockExtension
{
    public static string GetFormattedText(DependencyObject obj)
    { return (string)obj.GetValue(FormattedTextProperty); }

    public static void SetFormattedText(DependencyObject obj, string value)
    { obj.SetValue(FormattedTextProperty, value); }

    public static readonly DependencyProperty FormattedTextProperty =
        DependencyProperty.Register("FormattedText", typeof(string), typeof(TextBlockExtension),
        new PropertyMetadata(string.Empty, (sender, e) =>
        {
            string text = e.NewValue as string;
            var textBl = sender as TextBlock;
            if (textBl != null)
            {
                textBl.Inlines.Clear();
                Regex regx = new Regex(@"(http://[^\s]+)", RegexOptions.IgnoreCase);
                var str = regx.Split(text);
                for (int i = 0; i < str.Length; i++)
                    if (i % 2 == 0)
                        textBl.Inlines.Add(new Run { Text = str[i] });
                    else
                    {
                        Hyperlink link = new Hyperlink { NavigateUri = new Uri(str[i]), Foreground = Application.Current.Resources["PhoneAccentBrush"] as SolidColorBrush };
                        link.Inlines.Add(new Run { Text = str[i] });
                        textBl.Inlines.Add(link);
                    }                        
            }
        }));
}

Then in XAML we use it just like this:

<TextBlock local:TextBlockExtension.FormattedText="{Binding MyText}" FontSize="15"/>

And after putting some text to my property:

private void firstBtn_Click(object sender, RoutedEventArgs e)
{
    MyText = @"Simple text with http://mywebsite.com link";
}

I can see such a result: