Role based security

I'm using web forms. All I want to implement is a simple security check based on role when clicking a button to open a secured page.

I'm looking at the link you sent me on web forms and it seems to me it should be workable but I do have questions regarding implementing it.
The sample code that caught my eye is this...

public void Application_AuthenticateRequest( Object src , EventArgs e )
{
   if (!(HttpContext.Current.User == null))
   {
      if (HttpContext.Current.User.Identity.AuthenticationType == "Forms" )
      {
      System.Web.Security.FormsIdentity id;
      id = (System.Web.Security.FormsIdentity)HttpContext.Current.User.Identity;
      String[] myRoles = new String[2];
      myRoles[0] = "Manager";
      myRoles[1] = "Admin";
      HttpContext.Current.User = new System.Security.Principal.GenericPrincipal(id,myRoles);
      }
   }
}

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Basically here the roles are hard coded?

when I opened my Global.ASAX, I don't see this event..  Only application start
public void Application_AuthenticateRequest( Object src , EventArgs e )

You have to add it. And yes the example is very crude. Ideally roles and their definitions should come from a database and of you are implementing a really simple solution than web.config should be used to store the roles.

I use web forms as I mentioned. I would like to see how to set it up in web config and how to access it thru code and also eventually
how to use role definitions from a database.

In that case, I would recommend that you directly follow this article : https://docs.microsoft.com/en-us/aspnet/web-forms/overview/older-versions-security/roles/creating-and-managing-roles-cs

It has everything you have asked for. I know that the article is way old but if you are using WebForms then I think it is more than suitable to your requirements.

Please add the following article to above - it is actually a set of articles that will complete your requirements. At minimum do also look at: https://docs.microsoft.com/en-us/aspnet/web-forms/overview/older-versions-security/roles/role-based-authorization-cs

If you have a question or need detailed understanding please let me know.
PS: Do remember to read up on my original recommended reading of Security Concepts.

The answers were thorough

Thanks for your comments :)