I will not pretend this question does not have personal significance to me, but that need not to be overly elaborated.
By studying cases in which existentialists attempt to give advice to nihilists (of the intellectual as opposed to emotional or moral varieties), the argument seem to revolve around the concept that under the absence of objective meaning, one ought to construct meaning of one's own.
I wonder if this is the only central command of existentialism, with "existentialism" being defined as effectively a philosophical system with which a person can find one's action free and meaningful despite knowledge and beliefs that supports a nihilistic attitude.
(My personal motivation derives from the fact that I am not able to accept the notion that one must construct one's own meaning, because I believe such constructions are ultimately determined by biology and existing social conditions. Also, I find personal concepts of meaning to lack imperiousness of something like religious commandments or mathematical theorems, which seems to be the only magnitude that can satiate my appetite)