In what context? If you're trying to send someone to the admin area of a subaccount, then it would be https://[subdomain].instructure.com/accounts/[subaccount#] where the account # would correspond to the subaccount you want to send them to. Besides for the admin area for a subaccount, not sure there's really much else a normal user needs to worry about subaccounts for.
We are giving the Canvas Admins area a little bit of love (especially questions that are really, really old) and just want to check in with you. This will also bring this question new attention.
Were you able to find an answer to your question? I am going to go ahead and mark this question as answered because there hasn't been any more activity in a while so I assume that you have the information that you need. If you still have a question about this or if you have information that you would like to share with the community, by all means, please do come back and leave a comment. Also, if this question has been answered by one of the previous replies, please feel free to mark that answer as correct.