When SharePoint Hub site introduced it allowed register the site as Hub site using Power Shell only and While creating new site custom site design only allow to create associated site under hub site automatically or after create the site and associate it using site Information. In Ignite 2018 Microsoft announced new features using UI to create Hub sites and associate the hub site while creating new site.
Create associated site from within hub site itself
Make governing sites associated to a hub site easier by having them automatically associated and created from within the hub site itself. You’ll connect sites to hubs by using a new button right on the hub site itself. when you click it, you get our normal new site experience and the site will be automatically associated to the hub site.
Create, associate/disassociate, plus report on hub sites in the SharePoint admin center in Office 365
The SharePoint admin center in Office 365 will now allow you to create SharePoint hub sites and associated other sites once the hub site exists. This also provides a view into hub sites as they exist allowing for common admin tasks like reporting, contacting and managing sites – now including hub sites from the admin center user interface. The hub site association column also lets you see the sites associated with a hub.
Approval on association
A flexible approval process further empowers hub site owners and site owners to make requests and programmatically adapt to changes within their growing, dynamic intranet.