local start_button = nil --- replace your Start Button here 
local enter_min = nil --- replace your input box ID here 
local enter_sec = nil --- replace your input box ID here 
local display_time = nil --- replace your text here 
local mins = 0 --- replace the mins you want to countdown 
local secs = 0 --- replace the secs you want to countdown
local uiid = nil --- replace your UI ID here 

function enter_time_to_countdown_(obj) 
    if obj.uielement == enter_min then
        mins = tonumber(obj.content) or 0 -- Convert input to number
    end
    if obj.uielement == enter_sec then
        secs = tonumber(obj.content) or 0 -- Convert input to number
    end
    Chat:sendSystemMsg(mins .. " and " .. secs)
end

ScriptSupportEvent:registerEvent("UI.Lost.Focus", enter_time_to_countdown_)

function countdown(e)
    if e.uielement == start_button then
        repeat
            if secs == 0 and mins > 0 then 
                mins = mins - 1
                secs = 59
            elseif secs > 0 then
                secs = secs - 1
            end
            Customui:setText(e.eventobjid, uiid, display_time, mins .. ":" .. (secs < 10 and "0" or "") .. secs)
            Trigger:wait(1)
        until mins == 0 and secs == 0
    end
end

ScriptSupportEvent:registerEvent("UI.Button.Click", countdown)