SharePoint online: Add site collection administrator using PowerShell

Prerequisites: You need Microsoft SharePoint Online Management Shell installed on your machine.

Script:

 $loadInfo1 = [System.Reflection.Assembly]::LoadWithPartialName("Microsoft.SharePoint.Client")  
 $loadInfo2 = [System.Reflection.Assembly]::LoadWithPartialName("Microsoft.SharePoint.Client.Runtime")  
 $loadInfo3 = [System.Reflection.Assembly]::LoadWithPartialName("Microsoft.SharePoint.Client.UserProfiles")  
 #Must be SharePoint Administrator URL  
 $webUrl = "https://tenant-admin.sharepoint.com"  
 $username = "admin@tenant.onmicrosoft.com"  
 $password = "password"  
 $cred = New-Object -TypeName System.Management.Automation.PSCredential -argumentlist $userName, $(convertto-securestring $Password -asplaintext -force)  
 Connect-SPOService -Url $webUrl -Credential $cred  
 $site = Get-SPOSite -Identity https://tenant-my.sharepoint.com/personal/mysites_domain_com  
 Set-SPOUser -Site $site.Url -LoginName username@email.com -IsSiteCollectionAdmin $true