If the name I used on the publication is different from my legal name, which one should I use in the copyright form?
Between name changes, use names, transliteration, etc., this is a rather common situation, and copyright forms are generally well designed to allow graceful handling of it. Most copyright forms that I have encountered include two separate places where you put a name:


A list of the authors, in which you should write all of the names exactly as they appear on the paper.
A signature line, in which an authorized signer (in this case, yourself) should place their usual signature, in the same form that you use it for other legal documents.


There is no actual requirement that these names be identical, just that the authorized signer take legal responsibility for the assertions of the document, including asserting that the list of author names is appropriate.  In fact, for some authors, their organization may not allow the author to be the person who signs the document!

Thus, if you write your name on the paper as "author" and you write your legal name for signature, this should be fine and appropriate. If, somehow, you encounter a form that does not make the distinction between the two names, then I would say it is highly likely that the organization issuing the form must be rather sloppy with regards to handling copyright in any case, and you should not worry about it.