Create a remote event in ReplicatedStorage, I’ll call it “ApplyKnockback” in the example but you can name it whatever you want.
local Players = game:GetService("Players")
local ReplicatedStorage = game:GetService("ReplicatedStorage")
local LOCALPLAYER = Players.LocalPlayer
ReplicatedStorage.ApplyKnockback.OnClientEvent:Connect(function(kbVelocity: Vector3)
local character = LOCALPLAYER.Character
if not character then return end
local rootPart: BasePart? = character:FindFirstChild("HumanoidRootPart")
if not rootPart then return end
rootPart.AssemblyLinearVelocity = kbVelocity
end)
Then you can fire the target client on the server with the velocity you want.