Add-Type -Path "c:Program FilesCommon Filesmicrosoft sharedWeb Server Extensions15ISAPIMicrosoft.SharePoint.Client.dll"
Add-Type -Path "c:Program FilesCommon Filesmicrosoft sharedWeb Server Extensions15ISAPIMicrosoft.SharePoint.Client.Runtime.dll"

$siteUrl = “https://tenant.sharepoint.com/sites/mysitecollection”
$username = "username@tenant.onmicrosoft.com"
$password = Read-Host -Prompt "Enter password" -AsSecureString
$ctx = New-Object Microsoft.SharePoint.Client.ClientContext($siteUrl)
$credentials = New-Object Microsoft.SharePoint.Client.SharePointOnlineCredentials($username, $password)
$ctx.Credentials = $credentials

$webCreationInformation = New-Object Microsoft.SharePoint.Client.WebCreationInformation
$webCreationInformation.Url = "site1"
$webCreationInformation.Title = "Site 1"
$webCreationInformation.WebTemplate = "STS#0"
$newWeb = $ctx.Web.Webs.Add($webCreationInformation)

$ctx.Load($newWeb)
$ctx.ExecuteQuery()
Write-Host "Title" $newWeb.Title