Send Header from another domain in php

Well, I have a partner site, which would need to send its information to the client's browser as soon as the client accesses my site, I do it now with iframe and pixel, but it would be great if it were possible to do it in an automatic way directly when they access my site, as soon as iframe loads the whole page and leaves mine slower. I'm not sure what to do, but I'm not sure what to do. Even if it is not possible via header, any tips are welcome! ;) Anyway, thank you!