A salad that will be eaten in five minutes has different needs from one carried to lunch. Dressing starts changing the leaves and draws moisture from some vegetables while it waits.
Dry the washed ingredients well, use a suitable chilled container, and pack the dressing in a small leakproof pot. Mix just before eating. Beans and sturdy grains may tolerate earlier dressing better, so you can season those separately from the leaves.
Do not fill the box so tightly that there is no room to toss. A slightly larger container can make lunch much less awkward. If there is no space for separate pots, put the wettest ingredients away from delicate leaves and expect some texture change rather than promising an all-day crisp result.
Keep the whole prepared meal at an appropriate safe temperature during storage and transport.