Batch command to open url and fulfill username and password
I don't think this can be done using a batch file as batch has its own limitations, instead you can you the below VB script.
Set IE = CreateObject("InternetExplorer.Application")
IE.navigate "http://TheWebsite"
IE.Visible = True`
While IE.Busy
WScript.Sleep 50
Wend
Set ipf = IE.document.all.username
ipf.Value = "Username"
Set ipf = IE.document.all.password
ipf.Value = "Password"
Set ipf = IE.document.all.Submit
ipf.Click
IE.Quit
Update Website name, uname and passwd and then save this as AutoWebsite.vbs