I want to create a custom web part with a button that will provision a new site based off a pre-designed site template

I want to create a custom web part with a button that will provision a new
site based off a pre-designed site template

How do I connect the pre-designed site template to the create site class
and then call that class in the button1_click event action?
protected void Button1_Click(object sender, System.EventArgs e)
{
Server.Transfer("new site based of pre-designed site template", true);
}