Can I use my own Login page instead of discourse’s Default login dialog box ?
for example login.example.com
Maybe you want DiscourseConnect - Official Single-Sign-On for Discourse (sso)?
Do you want to change where people log in, or what it looks like.
Sorry for the late reply 
But I want to redirect the user to my own login for example login.appzard.com…
for logging in to community.appzard.com 
When you set up SSO for your discourse (discourse as a client) all login requests are directed to your auth server. So if you set up SSO correctly, this should be the expected behavior.