subdomain

I don't understand how to setup a site as a subdomain. I have the network setup and I added a site but I cannot figure out how to login to it.
The main site is marigoldmama.com
The subdomain is https://www.lemongrassboutique.marigoldmama.com

I would like for the user of this subdomain to be able to login to that section and add products etc.

Please advise.
Thanks in advance.