$cred = Get-Credential
Invoke-WebRequest -Uri 'https://mysite' -Credential $cred

$user = "user"
$pass = "password"
$securepasswd = ConvertTo-SecureString $pass -AsPlainText -Force
$cred = New-Object System.Management.Automation.PSCredential($user, $securepasswd)

Invoke-WebRequest -Uri 'https://mysite' -Credential $cred