As of now, its not possible to prefill a Widget. As a workaround, you can save the default value and mark it as read only in text field. While creating widget, drag and drop the text field and double click it. Then, you can provide default value and can check "Read Only" and save.
So if we collect the fields in our website for a w-9 as an example, then we can pass the fields when creating the widget using mergeFieldInfo. Right?
That way we can default the a tag document to the values passed. We mark those fields as read only and then customer will only need to esign. is that correct?
Correct in regards of using mergfieldInfo. and tags.
prefill is a specific term referring to manually (pre) filling a form, which is indeed not possible with widgets.
You are referring to mergeing formfield data from another system, which is perfectly possible using the mergfieldInfo parameter.
in terms of tags the main concern is the name of the field and whether or not you set the field read only for the signer.