As a referee, is it appropiate to ask for further details on a model extracted from another work? I'm reviewing a work in applied mathematics, and on it, the authors work on an equation that is derived from a previous work.

There is a parameter that has been chosen in a way I would like to be justified. Looking into the reference where they extracted the equation, the original authors also don't give any explanation for it.

Therefore, should I ask for further details on this parameter? or is it enough for them to point out the seminal work?
The onus is on the authors to appropriately justify all of their choices and conclusions, either directly or by referring to previous work which in turn contains an appropriate justification.

In this case they have referred to previous work which (in your view) does not contain an appropriate justification, so their job is not done.  It is completely reasonable for you as referee to object.

(Even if the previous work did contain appropriate justification, you might still feel, as in Buffy's answer, that the justification should be written in the present paper, to make it more self-contained.  That's also appropriate to mention as a reviewer, but it's more a matter of style than correctness.)