Find time of highest and lowest candles

You can find the high using highest and low using lowest function. Then you can find the index of them using barssince function. Then you can use them to plot a box at the last bar using barstate.islast. Example below

//@version=5
indicator(title="Highest Lowest Bar", shorttitle="HLB", overlay=true)

hi=ta.highest(high,20)
lo=ta.lowest(low,20)
hiIndex=ta.barssince(high==hi)
loIndex=ta.barssince(low==lo)
var box b=na
if barstate.islast
    box.delete(b)
    b:=box.new(bar_index-hiIndex,hi,bar_index-loIndex,lo,bgcolor=color.new(color.blue,80),border_color=na)