@dor, it’s not just DOM load in general but rather the form itself. It really depends on how the form is loaded. If say it is done with an AJAX call at some point in time after DOM of the page has been rendered your approach might still not work.
If you are in control of the form loading process you just need to call
enableFormTracking on acknowledge the form has been loaded. If you are not in the control of that process (say, due to 3rd party involved loading the form for you) you might need to implemented your own piece of code checking the presence of the form and then calling
Again, to know the answer to your question you need to know how the form is being loaded (inserted) into the page.