$user = "api_user"
$pass = "api_pass"
$secpasswd = ConvertTo-SecureString $pass -AsPlainText -Force
$credential = New-Object System.Management.Automation.PSCredential($user, $secpasswd)

Invoke-RestMethod -Uri 'http://api/sms' -Method Post -Credential $credential -Body '{from: "Sender",  to: "My_Number",  body: "my text",  messageType: "Normal",  requireDeliveryNotification: false,  validFor: 60,  charset: "Default" }'