Problem with automating Logon to a web site


I’m trying to automate logging onto a web site and I can get the username and password entered with the following code:

$ie=New-Object -Comobject 'InternetExplorer.Application

$usernamefield = $ie.document.IHTMLDocument3_getElementByID(‘username’)

$usernamefield.value = $username …

Where I am having an issue is with sending a click to the Login button.If I use by element or ID, it returns a null, I’m thinking it’s not finding the button?

The html is in the following screenshot.