My problem:
I want to create a new form and have it automatically linked to an existing spreadsheet.
It can create a new sheet on the existing spreadsheet, but the linking step should be automatic.
Unfortunately, it seems that a copy of a form does not auto-link to the same spreadsheet as the original form, but if there is a solution based on copying forms rather than creating, that works as well.
Best Answer
To automatically link a new form to an existing spreadsheet, create the form from the spreadsheet rather than from https://forms.google.com
Note: This method allows to add multiple forms to a spreadsheet the same way that it's possible to send the responses from the form editor to an existing spreadsheet even if that spreadsheet already have linked a form.