Invoke-WebReqeust - XML String needs to be stored in a form field called REQUEST and should be sent to our servers via HTTP POST method - powershell

Question

Stuck following some poor documentation. Attempting to connect to a vendors API to pull orders into my own DB. The vendor has only one line and example (not even a proper url that I am still attempting to get).
Documentation =
XML String needs to be stored in a form field called REQUEST and should be sent to our servers via HTTP POST method. All of the methods described in this document must follow this standard.
Example =
GET ORDERS method XML format:
<?xml version="1.0" encoding="utf-8"?>
<OrderXML>
<Method>GET ORDERS</Method>
<Authentication>
<Username>Your Username</Username>
<Password>Your Password</Password>
</Authentication>
</OrderXML>
Tried in Postman and PowerShell just cannot figure out how to get "Request" form.
My code returns bad request and I am not sure if due to URL or not getting the "Request" form coded correctly.
$URI = "https://orders.website.com/webservice/default.cfm"
[xml]$requestXML = #'
REQUEST=
<OrderXML>
<Method>GET ORDERS</Method>
<TestMode>YES</TestMode>
<Authentication>
<Username>USERNAME</Username>
<Password>PASSWORD</Password>
</Authentication>
</OrderXML>
'#
Invoke-WebRequest -Uri $URI -Method Post -Body $requestXML
Thank you

Answer 1

So finally figured it out in Postman.
Used the form control instead of Raw XML.
With Key being the required "REQUEST" and the Value being the XML.