Русско-англо-русский юридический словарь

[...] require the agent to act on the principal's instructions and at the principal's request to institute or defend proceedings, or to do whatever else is necessary to maintain the validity and enforceability of the principal's rights; (Commercial and Intellectual Property Law and Practice 2018 by Kier Bamford)

A person who affixes the signature of another to a document with permission and/or upon the principal's instructions is called an "amanuensis" (a "legal Latin" term). (LawGuru)