This procedure will enable HTTPS access to HTTP REST API and web user interface of the product but using self-signed certificate. It's an easy way but at least with 2 drawbacks:
/etc/init.d/lighttpd gen_certificate
cp /etc/lighttpd.d/01-ssl-selfsigned.conf.sample /etc/lighttpd.d/01-ssl-selfsigned.conf
/etc/init.d/lighttpd restart
This procedure will enable HTTPS access to HTTP REST API and web user interface of the product with a full chained certification. It's the most secured way but you need to have a DNS entry pointing to your product and the associated certificates provided from a Certificate Authority (like Let's Encrypt).
mkdir -p /etc/lighttpd/certs/
cat cert.pem privkey.pem > pkcert.pem
cp /etc/lighttpd.d/01-ssl-fullchain.conf.sample /etc/lighttpd.d/01-ssl-fullchain.conf.conf
/etc/init.d/lighttpd restart