Oh now i understand you.
I can assume you have switch-panel.myserverdomain.com where snappymail should be at switch-panel.myserverdomain.com/snappymail
Now I see you created webmail.myvercelwebsite.com. This will need you to have the following rewrite rules on webmail.myvercelwebsite.com since this where we want to redirect:
### Rewrite Rules Added by CyberPanel Rewrite Rule Generator
RewriteEngine On
RewriteCond %{HTTPS} !=on
RewriteRule ^/?(.*) https://%{SERVER_NAME}/$1 [R,L]
### End CyberPanel Generated Rules.
RewriteEngine on
RewriteCond %{QUERY_STRING} ^$
RewriteRule ^$ https://switch-panel.myserverdomain.com/snappymail? [R=301,L,NC]
Tested and it works