Используйте этот код:
private void button1_Click(object sender, EventArgs e)
{
// insert message to database
if(richTextBox_TextToSend.TextLength>0) {
string txt = richTextBox_TextToSend.Text;
int start = richTextBox1.TextLength;
string newMessage = from + ": " + richTextBox_TextToSend.Text + Environment.NewLine;
// send the typed message
sendMessage(from,to,task_id,txt);
// show the typed text in the richTextBox1
richTextBox1.AppendText(newMessage);
richTextBox_TextToSend.Clear();
richTextBox1.Select(start, from.Length);
richTextBox1.SelectionFont = New Font(richTextBox1.Font, FontStyle.Bold);
}
}