Keyboard Functions C#

You can get the focused TextBox using Control.Focused[^] property. See this[^] example.

[UPDATE]
My guess is that you want to append text to the TextBox that is focused. You could write your own helper method say void AppendText(string strToAppend) that checks for the active TextBox and append the given string to it.

For example if you have 2 TextBoxes (textBox1 and textBox2) than you could write get focused TexBox method like this:

private TextBox GetFocusedTextBox()
{
    if (textBox1.Focused == true)
    {
        return textBox1;
    }
    if (textBox2.Focused == true)
    {
        return textBox2;
    }

    // Here you can add more...

    // There is no focused textbox
    return null;
}

If you have lots of textboxes then you can change the GetFocusedTextBox() method to iterate through the controls on the form using the first example from the link that I gave you.
Your append text method could be something like:

private void AppendText(string strToAppend)
{
    TextBox tbFocused = GetFocusedTextBox();
    if(tbFocused != null)
    {
       tbFocused.Text += strToAppend;
    }
    else
    {
       // There is no focused textbox, you should decide what to do
    }
}

This is an easy solution and you can call the AppendText method where you want to append text to the focused TextBox. I hope this helps. Feel free to post a comment in case this not enough or not clear. :)
[/UPDATE]

[UPDATE 2.1]
Sorry for late answer, my internet connection was down but I'm here with a working example for you. :)
First I will paste the code and after that will try to explain:

public partial class Form1 : Form
{
    // These are members that we will use to restore
    // the selection of last selected textbox,
    // restore the caret position and selection length
    private TextBox tbSelected; // Last focused TextBox
    private int posCaret;       // Caret position
    private int selLength;      // Selection length
    public Form1()
    {
        InitializeComponent();
        // We will use leave event for textboxes
        textBox1.Leave += new System.EventHandler(textBox_Leave);
        textBox2.Leave += new System.EventHandler(textBox_Leave);
        // Set initial selection to the first textbox
        textBox1.Select();
        tbSelected = textBox1;
        posCaret = 0;
        selLength = 0;
    }
    private void button1_Click(object sender, EventArgs e)
    {
        // Call this to restore the last selection
        RestoreLastSelection();
        // Set the selected text
        tbSelected.SelectedText = "S";
    }
    private void button2_Click(object sender, EventArgs e)
    {
        // Call this to restore the last selection
        RestoreLastSelection();
        // Set the selected text
        tbSelected.SelectedText = "FOO";
    }
    // Leave event handler
    private void textBox_Leave(object sender, EventArgs e)
    {
        // Remember the last focused thextbox,
        // the caret position in it and the selection length
        tbSelected = (TextBox)sender;
        posCaret = tbSelected.SelectionStart;
        selLength = tbSelected.SelectionLength;
    }
    // Helper method to restore selection
    private void RestoreLastSelection()
    {
        tbSelected.Select();
        posCaret = tbSelected.SelectionStart;
        selLength = tbSelected.SelectionLength;
    }
}

I've created a new project (Windows forms application) added 2 buttons and 2 textboxes. My approach is very simple. Have a look at the Control.Leave[^] event. I use this event to register the last focused textbox, caret position in it and the length of the selected text. After a button click occurs I restore the selection in the textbox and then replace the selected text. This way you can add text to cursor position and/or replace selected text. Examine the source above. I believe that this approach is easy to understand and use. Give it a try and let me know if there are some problems. :)
[/UPDATE 2.1]