Adding a private site for internal company use

[Erobinson1]

We have a Shopify site we use for retail orders but we would like to create a separate private site to use within our company. We would like this to be a private subdomain for our employees to be able to order quantities within this private site. Is there a way to do this? We have an advanced Shopify plan and I am unable to add a subdomain. I go to Settings>domains>click our primary domain> there is no option from there to add a subdomain. If someone has an answer for this issue that would be great. Thanks!