You shouldn’t need to add anything to .htaccess. Assuming your subdomain points to the same place on the filesystem as the main domain then you need to configure WP to accept requests to both hostnames (to prevent a canonical redirect – which is what I assume you were experiencing?).
In other words, instead of hardcoding the domain name in WordPress (WP_SITEURL and WP_HOME), you need to make this dynamic based on the requested hostname.
For example, in wp-config.php set the following:
define('WP_SITEURL','https://'.$_SERVER['HTTP_HOST']);
define('WP_HOME','https://'.$_SERVER['HTTP_HOST']);
RewriteCond %{HTTP_HOST} ^(.*)\.example\.com$ RewriteRule ^(.*)$ http://example.com [P]
This is attempting to “proxy” the request from the subdomain to your main domain – using your server as a reverse proxy. This requires access to your server config to configure (properly).