Please create a new .htaccess file in your site root folder and place the below code in it:
RewriteEngine on
RewriteCond %{HTTP_HOST} ^(www.)?example.com$
RewriteCond %{REQUEST_URI} !^/subdirectory/
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^(.*)$ /subdirectory/$1
RewriteCond %{HTTP_HOST} ^(www.)?example.com$
RewriteRule ^(/)?$ subdirectory[L]
Please make sure that you substitute example.com for your primary domain and subdirectory for the subdirectory to which you are redirecting the domain.
