Hi there! As a former German and English teacher and presently working for a research foundation, I have been in the same situation. In American English, you may write "for" after signing your name. www.askoxford.com, the British "proper" style is using the abbreviation, "p.p.", per procurationem, from Latin, meaning 'through the agency of'. Again, you would use these initials after signing your first and last name.
Great question!