You have to have bought the golden squirt gun (check the shopping site on the PC in your office) then the next time the water gun fight event triggers you will automatically be like oh hey actually I have my own gun wink wink and you will put the formula shit in automatically too if I recall correctly.Problem is: i cannot use the formula, because the game doesn't allow me to use the formula from inventory. There's a specific moment where i can use it?
But again - unless I am remembering wrong - you cannot even buy the squirt gun until AFTER you have played the water gun fight with the girls once using the shitty gun they give you.