Automatically fill dispensers in Minecraft?

I'm looking for a command with which you can automatically fill in Minecraft Dispenser (which is never an instruction).

Which version?

1.15.2

Which item should be in the launcher?

Throwing potion

And what throwing potion?

Try https://mcstacker.net/. There you can select at / setblock dispenser and then select the item in the slots. Then use a clock to execute the command or simply set the command block accordingly.

Poisoning

Ok, and in which direction does the thrower point?

Is that automatically filled with water?

/ data merge block ~ ~ ~ {Items: [{Slot: 0b, id: "minecraft: splash_potion", Count: 1b, tag: {Potion: "minecraft: swiftness"}}]}

Simply enter coordinates and potion

West

Thank you.

It always replaces the block with the full dispenser

You write in a command block (repeat | unconditional | always active):

/ setblock coordinates of the thrower minecraft: dispenser [facing = west] {Items: [{Slot: 0b, id: "minecraft: splash_potion", Count: 1b, tag: {Potion: "minecraft: poison"}}}]}