Whether or not "ID attribute" can be added to submission button of Marketo form

Hi! Changing/setting the ID won't affect the values sent to the server.  Your partner may be confusing a Marketo form for a native HTML form, where altering the name and value of a submit button will set an additional form field to be sent to the server (usually, this is used to distinguish two different actions that can result from the same form, such as "Delete Contact" or "Update Contact").  Marketo forms don't work this way.

If you want to use the the same form but add a "submission type" depending on the page you're on, just use addHiddenFields:

MktoForms2.whenReady(function(form) {             
form.addHiddenFields({                 
  'LastSubmitButtonAction': 'news_letter'             
});         
});

Of course, you will have to add a custom field in your instance to hold this value.

Note that while there's nothing wrong with this approach, most users distinguish different submissions of the same form based on the URL that hosts the form (assuming, naturally, that a single URL will never use the same form for different purposes).  When you do it this way, you don't need to create a field to record the different events.