What is the proper way to pass a list to a function taking only a flattened list?

Question

In scheme, functions can return single values :

(lambda () 5)

or can return lists :

(lambda () '(1 2 3))

My question is about composition.

You can combine functions like this :

(+ 1 (lambda () 2))

but, What is the proper and efficient way to pass a list to a function that only takes a flat list as argument ?

This is what I'm trying to do, but is not specific to the + function :

(+ (lambda () '(1 2 3)))

or simply

(+ '(1 2 3))