Use these commands to create the new template and use it on your web domain (replace YourUser and YourDomain with the actual data):
cd /usr/local/hestia/data/templates/web/nginx/php-fpm/
cp wordpress.tpl wordpress-http3.tpl
cp wordpress.stpl wordpress-http3.stpl
sed -i '/^server {$/a \\tlisten %ip%:%web_ssl_port% quic;' wordpress-http3.stpl
sed -i '/if (\$anti_replay = 425) { return 425; }/a \\n\tadd_header Alt-Svc '\''h3=":$server_port"; ma=86400'\'';' wordpress-http3.stpl
v-change-web-domain-tpl YourUser YourDomain wordpress-http3 yes
Instead of running the last command v-change-web-domain-tpl, you can edit the web domain through the Web UI by changing the template used by your site there.