You may use this snippet, which finds a text from textBox1 inside TextBox2 by clicking on a button:
private void button1_Click(object sender, EventArgs e)
{
int idx = textBox2.Text.IndexOf(TextBox1.Text);
if (idx >= 0)
{
textBox2.SelectionStart = idx;
textBox2.SelectionLength = TextBox1.Text.Length;
textBox2.Focus();
}
}