SharePoint 2013 - Can not get site (web appication) to use FQDN without the path "/sites/main"

I have a new SharePoint 2013. The site works fine with the URL https://sharepoint.mydomain.com/sites/main However I want users to be able to just enterhttps://sharepoint.mydomain.com only. I have tried using the SPSiteURL command but I get an error and I'm not sure if thats the method I should use anyway, if this is wrong or if there is a better way.

The syntax I used is:

$site = Get-SPSite 'https://sharepoint.mydomain.com/sites/main'

Set-SPSiteURL -Identity $site -Url https://sharepoint.mydomain.com -Zone Default

get the following error:

Set-SPSiteURL : Could not set URL for the site collection
https://sharepoint.mydomain.com/sites/main. This command can only be
run on a host-named site collection.
At line:1 char:1
+ Set-SPSiteURL -Identity $site -Url https://sharepoint.mydomain.com
-Zone  ...
+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
~~~
    + CategoryInfo          : InvalidData: (Microsoft.Share...letSetSPSiteUrl:
   SPCmdletSetSPSiteUrl) [Set-SPSiteURL], ArgumentException
    + FullyQualifiedErrorId : Microsoft.SharePoint.PowerShell.SPCmdletSetSPSit
   eUrl

Thank you for any assistance!