Domain/ Hosting Setup for Multisite

I running an existing WordPress site which I have had for many years. I am looking to bolt on an LMS system using WordPress multisite and have therefore set up Network Settings to do this.

I have added this as a Site in WordPress however, when I navigate to the URL administration of the second site it does not work.

My understanding is that this should be sharing the same WordPress installation as my current site therefore I do not think I need install WordPress on my second domain?

I have set up the domain in my hosting panel as a subdomain but do I need to do anything further? Or do I need to change any DNS records?

Any help would be appreciated.

Regards,

James