How to create a new host named site collection in its own content database using Powershell

Step 1:  Add New Content Database to the Web Application
New-SPContentDatabase -Name <content_db_name> -WebApplication <web application URL>

Step 2: Get a reference to the hostnamed web application:

$webApp = Get-SPWebApplication "MyHostNamedWebApp" 

Step 3: Create a site collection 

new-spsite http://sharepointDev.com -ownerAlias "domain\user" -HostHeaderWebApplication $WebApp -ContentDatabase "WSS_Content_Dev_test" -template sts#0