Change your configuration to this (add a slash):
RewriteCond %{HTTP_HOST} ^example.com$ [NC]
RewriteRule (.*) http://www.example.com/$1 [R=301,L]
Or the solution outlined below will work for any domain:
RewriteEngine On
RewriteCond %{HTTP_HOST} !^www\. [NC]
RewriteRule ^(.*)$ http://www.%{HTTP_HOST}/$1 [R=301,L]
If you need to support http and https and preserve the protocol choice try the following:
RewriteRule ^login\$ https://www.%{HTTP_HOST}/login [R=301,L]
Source :
Randall Hunt(2021), https://stackoverflow.com/questions/12050590/redirect-non-www-to-www-in-htaccess/36807965