Your options are to add a print button to the form using Form.addButton from a user event script before load entry point or Custom Action on your record’s form.

The pure suitescript option would have you open a new window to a suitelet that writes a file generated via N/render. If you are willing to go through the javascript of netsuite’s print button, you can probably figure out how it generates the url to do its printing and generate that url yourself instead of making your own suitelet. I personally would expect making your own suitelet to to be easier that going through NetSuite’s code.