We are going to use the acme.sh script to accomplish this. For authentication of the domain name, we will use the DNS option. First login to your Synology with ssh as the admin user and then sudo -i to get root access. When you login into the Synology with ssh you will end up in the /root path. I assume for the rest of the guide we run everything from that path. Now we need to get the script and change the permissions so it is executable.
wget https://raw.githubusercontent.com/Neilpang/acme.sh/master/acme.sh
chmod a+x acme.sh
Next we issue the certificate:
./acme.sh --issue -d *.website.come --dns --yes-I-know-dns-manual-mode-enough-go-ahead-please