As others have stated, your router would have to be configured to forward the appropriate ports to the computer with the shares. You will be opening up a huge security hole, so consider other options.
Almost anything will be better than SMB/CIFS over the open internet. Personally I'd use SCP/SFTP to take advantage of SSH security. You could even tunnel SMB/CIFS over SSH... in theory. The only thing you'd need to do is 1) setup an SSH server on your computer, and 2) forward SSH ports from your router to the computer.
Now, to your question.
smb://dyndnsalias.com/HostnameOfMyPC/Share won't work because you have two hostnames in there,
HostnameOfMyPC. In theory (and depending on where you were trying to use this), the format would be:
OR + host/ip + share
You can use any of these for the host/ip field: