To change a farming objects Pickup settings go to its blueprint, so for example the BP_TomatoPlant and in the Class Defaults find the Pickup category like the image below.
Here you can change this plants Yield settings, you can hover your mouse over each settings name to get a tooltip with more info about each setting.
If Can Pickup is true then this farming object can be picked up. The player will then receive the resources set in the Pickup Resource array. Multiple resources can be added here by clicking the + button, then just set the resource name and the amount of that resource that should be received when picked up.