If the task the employee is asked to perform on pain of firing is truly unreasonable (things like sexual favors or signing away essential legal rights), you could use the phrase (a) Hobson's choice. From Merriam Webster:
- : an apparently free choice when there is no real alternative
- : the necessity of accepting one of two or more equally objectionable alternatives
This expression has even been adopted into employment law for this type of situation; a typical description of the theory:
Under the Hobson's choice theory, an employee's voluntary quit will be considered a constructive discharge when an employer conditions an employee's continued employment on the employee's abandonment of his or her Section 7 rights and the employee quits rather than comply with the condition.
—Scheid Electric, in Decisions and Orders of the National Labor Relations Board: V. 355. January 15, 2010 Through September 30, 2010
Of course, if the task in question is an ordinary part of the employee's expected duties, then it's going to be remember something like a contractual obligation or perhaps free enterprise, and phrasing it as a request rather than an order is probably just going to be considered good manners. (Phrasing an order as a request when talking to one's children, on the other hand, is called poor strategy.)