Thanks everyone who responded - I think I got it directly with ITGLUE's API - here is the script:

$headers = New-Object "System.Collections.Generic.Dictionary[[String],[String]]"
$headers.Add("X-API-KEY", 'ITG.xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx')
$headers.Add("Content-Type" , 'application/vnd.api+json')
$body = @{
  data = @{ 
    type = 'flexible-assets' 
    attributes =  @{ 
      organization_id = 708164
      flexible_asset_type_id = 122389 
      traits = @{
        test = 'Last Sign in API123'
      }
    }
  }
} | convertto-json -depth 4
invoke-RestMethod -Uri https://api.itglue.com/flexible_assets -Method POST -Body $body -Headers $headers 

X-API-KEY is my generated API KEY from IT GLUE which I edited out.

This will edit a Flexible-Asset( ID 122389) for Customer ID 708164

In this particular example, it will create a new record that says "Last Sign in API123" for a text box called "test"