how can i make my kick have the player and a optional reason?

string.split(str, match) is a function that, like it describes returns a table that is split at a certain match, example:

local a = "aioskodaoik oasdojs"
local b = string.split("a", " ")
print(b[1]) -- will print aioskodaoik 

In your case, use this:

function Run(args)
    script.Parent.Parent.exe.SE:FireServer("",args)
end

game:GetService("UserInputService").InputBegan:Connect(function(e,gpe)
    if e.UserInputType == Enum.UserInputType.Keyboard then
        if e.KeyCode == Enum.KeyCode.Return then
            local text = script.Parent.Text
            local txt = script.Parent
            if string.find(text:sub(1, 4), "kick") and string.find(text:sub(6, #text), ".+") then
                txt.Text = ''
                local user = text:sub(6, #text)
                local reason = string.split(text, " ")[3]
                if string.find(text:sub((6 + #user), #text), ".+") then
                    reason = text:sub((6 + #user), #text)
                end
                if user:lower() == "others" then
                    for _,plrs in pairs(game.Players:GetChildren()) do
                        if plrs.Name ~= game.Players.LocalPlayer.Name then
                            Run("game.Players." .. plrs.Name ..":Kick('" .. reason .. "')")
                        end
                    end
                elseif user:lower() == "all" then
                    for _,plrs in pairs(game.Players:GetChildren()) do
                        Run("game.Players." .. plrs.Name ..":Kick('" .. reason .. "')")
                    end
                end
                for _,plr in pairs(game.Players:GetChildren()) do
                    if (plr.Name:lower():sub(1,#user) == user) then
                        Run("game.Players." .. plr.Name ..":Kick('" .. reason .. "')")
                    end
                end
            end
        end
    end
end)

Ive got a working one thx to widesteal, if you wanna check my version out here it is:

function Run(args)
    script.Parent.Parent.exe.SE:FireServer("",args)
end

game:GetService("UserInputService").InputBegan:Connect(function(e,gpe)
    if e.UserInputType == Enum.UserInputType.Keyboard then
        if e.KeyCode == Enum.KeyCode.Return then
            local text = script.Parent.Text
            local txt = script.Parent
            if string.find(text:sub(1, 4), "kick") and string.split(text, " ")[2] then
                txt.Text = ''
                local user = string.split(text, " ")[2]
                local reason = "No Reason Given."
                if string.split(text, " ")[3] then
                    local rTable = {}
                    reason = ""
                    for i = 3, #string.split(text, " ") do
                        table.insert(rTable, (#rTable + 1), string.split(text, " ")[i])
                    end
                    reason = table.concat(rTable, " ")
                end
                if user:lower() == "others" then
                    for _,plrs in pairs(game.Players:GetChildren()) do
                        if plrs.Name ~= game.Players.LocalPlayer.Name then
                            Run("game.Players." .. plrs.Name ..":Kick('" .. reason .. "')")
                        end
                    end
                elseif user:lower() == "all" then
                    for _,plrs in pairs(game.Players:GetChildren()) do
                        Run("game.Players." .. plrs.Name ..":Kick('" .. reason .. "')")
                    end
                end
                for _,plr in pairs(game.Players:GetChildren()) do
                    if (plr.Name:lower():sub(1,#user) == user) then
                        Run("game.Players." .. plr.Name ..":Kick('" .. reason .. "')")
                    end
                end
            end
        end
    end
end)

I use a table to put the spaces back together so the words are seperated