How do I make a user press down a key?

Required Knowledge:

• BindableEvents
• UserInputService
• KeyCode Enumeration

Probably the best approach to this would be making a BindableEvent and fire that when a key is pressed. You could then fire said event at will, effectively simulating a keypress.

Example:

local uis = game:GetService("UserInputService")
-- Set up the event
local event = Instance.new("BindableEvent")
event.Name = "KeyPressEvent"
event.Parent = script

uis.InputBegan:Connect(function(key)
    event:Fire(key.KeyCode) -- Fire the event
end)
event.Event:Connect(function(keycode) -- Use this from now on instead of UIS.InputBegan
    -- Your code
    if keycode == Enum.KeyCode.E then
        print("E was pressed")
    end
end)

event:Fire(Enum.KeyCode.E) -- Force a keypress

This is only an example and can be easily expanded on.

If I helped, be sure to accept/upvote!

local UserInputService = game:GetService("UserInputService")

UserInputService.InputBegan:Connect(function(Key,GameProcessed)
    if not GameProcessed then -- If Player Is Typing For Example, It wont do anything
        if Key.KeyCode == Enum.KeyCode.Q then -- Checks if the key is Q
            -- Run Your Code Here
        end
    end
end)

Or The WIKI