HTTP Authentication Using APM

I am working as a IT Consultant for a Firm. They are using old Microsoft TMG Server to proxy into some Windows IIS Backend applications. When we try to access the website from public internet, we will get a TMG login, we can input our AD credentials and login to the page. We are migrating TMG to F5. I have integrated F5 to the AD and enabled APM for getting the login page.

Now when we try to access the page, F5 login page comes insted of TMG. When I put in my AD credetials it log me in to a another basic authentication prompt where I have to put in my credentials again. TMG server logins directly to the application. The backed server is enabled with basic authentication from inside DMZ network, from public network TMG login is enabled. F5 is taking me to the HTTP login prompt. How to fix it. The backend server has forms based authetication enabled. I have made APM policy auth AD authentiation and SSO mappings.