Manage external domain redirect pointing to my server

Hello, I don't know if this post is in the correct section, but I need your help. I have a problem which is as follows. I have a business partner who has made a subdomain available for me to host the application. His subdomain is sistema.compania.com.br which is owned by him. I have another domain called meusistema.com.br that is hosted by the application. The client wants to use his subdomain and point to application. In other words, when typing sistema.companhia.com.br, open the system that is in the meusistema.com.br domain, but the name of the second domain does not appear. The trading partner made the DNS subdomain pointing to my server IP. However, when accessing the subdomain it is showing the error: "Sorry, It is possible you have reached this page because...". Is there any solution for me to use his subdomain that works for my system? I would like you not to use my domain and only use his subdomain. I appreciate the help.