POST https://{server}/restapi/{apiVersion}/oauth2/token
Accept: application/json
Content-Type: application/x-www-form-urlencoded
grant_type=password&client_id={IntegratorKey}&username={email}&password={password}&scope=api

curl -i -H "Accept": "application/json","Content-Type": "application/x-www-form-urlencoded", Content-Length": 152 -X POST -d "username=email%gmail.com&password=password&client_id=client_id&grant_type=password&scope=api" https://demo.docusign.net/restapi/v2/oauth2/token

Invoke-WebRequest : Missing an argument for parameter 'InFile'. Specify a parameter of type 'System.String' and try