If I want to make frozen homemade pizza, do I have to cook the toppings first? I am planning in using onions, bell peppers, carrots, tomatoes, salmon

If your salmon is fresh, adapt from this recipe (cut in thin pieces, bake 10 min)
https://www.ricardocuisine.com/en/recipes/4521-stromboli--s-fresh-salmon-pizza-
If your salmon is smoked, a la Wolfgang Puck, bake the pizza without the salmon, and then add it when pie is removed from oven.

I agree with Ryan. Additionally, and especially if you plan on making a thin crust pizza, don't crowd your toppings if you want them to cook.