How simulate CTRL+V keystrokes (paste) using C#

Character vs key

% => alt , + => shift and ^ for to send ctrl key

Original Answer:

Simulation of single modifier key with another key is explained below Step1: Focus the textBox, on which you want to perform two keys and then Step2: send the key for example control-v will be sent like "^{v}". Here is the code

target_textBox.Focus();
SendKeys.Send("^{v}");

target_textBox.Focus(); is needed only when target textbox is not focused at the time of sending key

Update: For sending three keys (two modifying keys plus other key) like to achieve ctrl shift F1 you will send following

^+{F1}

Microsoft Docs Ref

Why don't you override the TextBox OnClick event than when the event is called, set the Text property to Clipboard.GetText()

Like:

private void textBox1_Click ( object sender, EventArgs e )
{
        textBox1.Text = Clipboard.GetText ();
}