Remote login to Buddypress subfolder from top level WordPress homepage

I have a Buddypress installation on a subfolder of the primary site, and we’re trying to set up a login form in the primary site’s header.php on the top-level domain. Right now the form just kicks you to the WordPress login screen on the Buddypress site, without transmitting the username and password and actually logging in. Is there a way to make it log the user in?

My Googling around on this led me to this – https://www.ads-software.com/support/topic/remote-login-using-curl – but I”m not really a programmer, so I have no idea where to enter that Ajax POST string.

Thanks!