Hello @garethw - welcome to the forums!
I had a quick look at the web login for autotask, and you won’t be able to autofill that one. That is a very cleverly designed web page that seems to deliberately block autofill operations. If you inspect the login text box, the id it uses is randomly generated each time you load the page, so I don’t think there is a way to autofill it.
If you are a customer, I would contact their web support and make a complaint that they are blocking password managers. I suspect this is not their real intent, and their unconventional web form is actually some type of solution to impede bots from trying to login to their site.