There are two workflow variables, one contains site url and the other contains person details. The user account should be added in site owners group of that particular site.
Hi Dilip Pottendla,
You can have $site for site URL and $user for person details. And use below commands to update the site collection owners:
$web = get-spweb $site.url
$NewAdmin = $web.EnsureUser("domain\$user")
$NewAdmin .IsSiteAdmin = $true
Retrieving data ...