Hi, ScIT :
except Debian Buster, Hestia and Wordpress, I haven’t installed anything. It’s a fresh VPS.
I just need to change the endpoint of a domain declared in Hestia to ask Apache to redirect the entry request and routing to another one.
/home/hestia_user/web/Hestia_domain/public_html
ok guys, I give up and I leave Hestia. It takes too much linux geek knowledge. If doing a simple redirection becomes hell, I don’t dare to imagine what happens next.
The project still seems nice, I’ll come back in a few months, because Wordpress will always exist !
Thanks to all those who tried to help me.
Infact you’re using apache2, there is no reason for a custom template: Add your second domain to “alias” under edit web domain, after that, add the rewrite rule to your htaccess file and all should be done. Just in case you want to give it a try 
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@Raphael hi 
I tried this “almost” it works yes for wordpress : if I put domain_b in alias_area from domain_a (WordpressMasterMU), domain_b is well redirected to domain_a
But other problems will come ; ssl certificats first for domain_b and what happen if I need to deploy mails boxes for domain_b > mail.domain_b
Yesterday I also tried a 301 redirection from htacess in domain_b /public.html to redirect to domain_a. Completely ignored > still endpoint “We Working on it” page 
ssl isn’t a problem, because if you use letsencrypt, it will simply get you a multi-domain cert that works for all domains including the alias ones.
mail isn’t a problem, because mail domains are handled separately and have nothing to do with the web domain aliases. just add your second mail domain and create your mailboxes as with the first domain 
tl;dr; keep it simple instead of trying to bend the system in a way that’s neither intended nor needed 
have you seen my post on getting wordpress mu working on hestacp?