One key idea in contemporary Trinitarianism is the idea that the Son is eternal. This is required to have 3 co-equal persons (if the Son is not eternal, He is not equal to the Father).
Some try to show that the Son is eternal indirectly - the most obvious example is showing that the Son is God, and therefore the Son is eternal.
Even passages which might suggest pre-existence (such as John 8:58, "Before Abraham was, I am") don't directly impinge on the idea of eternally existing. There are all sorts of situations where someone existed before something, but aren't eternally pre-existing.
So, what is the direct scriptural evidence for the Son not just pre-existing but pre-existing eternally?
is similar, but too broad. For example (from an answer to the linked question), "The main evidence for the biblical basis of the Nicene interpretation is how the NT authors applied the term θεός" is not what I'm looking for.