When I use FormsAuthentication.SetAuthCookie(txtEmail.Text, True)
I know that it
Overloads Public Shared Sub SetAuthCookie( _
   ByVal userName As String, _
   ByVal createPersistentCookie As Boolean _
)

So is the cookie name txtEmail.Text? or how can I name my cookie? I don't want to create cookie except this method.

In web.config where you configure forms authentication you can provide the cookie name:

<authentication mode="Forms">
    <forms loginUrl="logon.aspx" name="cookiename" />
</authentication>