How to make a TFS Git pull request without the UI

$repository = "thereponame"
$sourceBranch = "feature/helloworld"
$targetBranch = "develop"
$reviewerId = "18ff8b6a-6806-4580-b263-4267cd79f86f"
$teamProject = "myteamproject"
$collection = "mycollection"
$tfsBaseUrl = "http://myserver"
$apiVersion = "3.0-preview.1"

[uri] $url = "$tfsBaseUrl/$collection/$teamProject/_apis/git/repositories/$repository/pullRequests?api-version=$apiVersion"

# Prepend refs/heads/ to branches so shortened version can be used in title
$sourceRefName = "refs/heads/$sourceBranch"
$targetRefName = "refs/heads/$targetBranch"

# JSON for creating PR without Reviewer specified
$body= @"
{
    "sourceRefName": "$sourceRefName",
    "targetRefName": "$targetRefName",
    "Title": "Merge $sourceBranch to $targetBranch",
    "Description": "PR created from powershell via REST API",
    "Reviewers":[{"id":"$reviewerId"}]
}
"@

# Use URI and JSON above to invoke the REST call and capture the response.
$response = Invoke-RestMethod -Uri $url `
                              -UseDefaultCredentials `
                              -Method Post `
                              -ContentType "application/json" `
                              -Body $body  

Write-Host "PR Id  : " + $response.pullRequestId
Write-Host "PR Url : " + $response.url