$AdminURL = "https://tenant-admin.sharepoint.com/"
$AdminName = "admin@tenant.onmicrosoft.com"
$NewUser = "newuser@tenant.onmicrosoft.com"

#User Names Password to connect
$Password = Read-host -assecurestring "Enter Password for $AdminName"
$Credential = new-object -typename System.Management.Automation.PSCredential -argumentlist $AdminName, $Password

#Connect to SharePoint Online
Connect-SPOService -url $AdminURL -credential $Credential

$Sites = Get-SPOSite -Limit ALL

Foreach ($Site in $Sites)
{
    Write-host "Adding Site Collection Admin for:"$Site.URL
    Set-SPOUser -site $Site -LoginName $NewUser -IsSiteCollectionAdmin $True
}