@smiha0, you can either fill the jar vase with harvested flowers from your sims’ inventory (drag from inventory), or you can get them in the debug catalog.
To access the debug catalog in build/buy mode, open up the cheat console (shift+ctrl+c), and enter: bb.showhiddenobjects. You can then search for any of the harvestable flowers (bluebell, daisy, orchid, etc.).
Place 3 flowers in the jar with bb.moveobjects on cheat. To remove the flowers, you’ll have to place the entire filled vase into the Household Inventory, which will separate the vase from its contents.