how to write medical claim form!

Dear all

I would like to summit my medical claim form to Finance department. One thing I have a problem with is I can't withdraw money by myself so I want one of my colleagues to withdraw on behelf of me..So my colleague need my authority letter . I don't know how to write it. Could you please help me how to write it if you have any idea?

thanks for your cooperation!

With best wishes!

Chaw Su