Validate Textbox Base On Drop Down Box?

I have a drop down list and a textbox on a form. If a user select employee in the dropdown box, I wish to add employee email addresses in the text box. It should do the same if users select client on the drop down (i. e. Add client addresses in the textbox). I have not been able to do it without having to click on a button.

Here is the code that should be executed:

protected void ddTemplate_SelectedIndexChanged(object sender, EventArgs e)
{
if (ddTemplate.Text == "Employees")
txtEmailAddress.Text = "employee@abc.com";
else if (ddTemplate.Text == "Clients")
txtEmailAddress.Text = "clients@abc.com";
}