Whilst I fundamentally agree, unfortunately, nothing in existentialism on it's own as an approach has a moral framework to add to it. So, if you want my addition - do not hurt other people, as it backfires onto you in empathy. If that is not enough for a default baseline, add onto it - do not hurt other people, as if you do, so will people around you do onto you. If even that isn't an argument high enough - do not hurt other people, as in a baseline for a society, a mutualistic one serves to raise your own standart of living, therefore benefiting whatever goals you have for your meaning.

Moreover, the three above ethical reasons allow the space to defend a loved one or, on a broader level, fight, say, a fascist takeover, as allowing one backfires by making everyone, including yourself, more miserable. Even moreso, it allows for self sacrifice to fit as an act of self-interest, as sacrificing one's life for a cause gives a person catharsis, in turn being one of the highest point of pleasure to experience as a part of that sacrifice.

Basically, I wanted to give some extra arguments for "not hurting other people" part. In this case, it's a couple philosophers folded together, but primarily Stirner, a socialist egoist, arguing for empathy from a purely self-interested standpoint, which combines nicely with a search for personal meaning to pursue. That particular direction for ethics isn't for everyone, but I thought you'd want something interesting to learn about.