This example shows the line at 3% below the last candle's close price.
I found this in one of my saved charts, which will hopefully give you a nudge in the right direction:
study(title = "Stop Loss", shorttitle="Stop")
// show a stop at -3%
stop = close * 0.97
plot(stop, color = red, trackprice=true, offset=-100000)
Hope that helps. :)
If you want to use this specific value in a calculation however (rather than the whole series of values), you're up a creek at the moment (until they add this functionality)...