Iterate over builds in VSTS
$personalAccessToken = "<your VSTS PAT Token>"
$tfsUrl= "https://myvsts.visualstudio.com/onecrm/_apis"
$headers=@{
Authorization = 'Basic ' + [Convert]::ToBase64String([Text.Encoding]::ASCII.GetBytes(":$($personalAccessToken)"))
}
$builds= Invoke-RestMethod -Uri $tfsUrl"/build/definitions" -Method Get -Headers $headers
foreach($build in $builds.value) {
$b = Invoke-RestMethod -Uri $build.url -Method Get -Headers $headers
$b.variables
}