HOWTO: alert when bar cross existing indicator

[Forex Trader]

Hello

I'd like to code an alert to be fired when price bar crosses an existing into the chart indicator, ie MA added through menu Insert -> Indicators -> Trend -> Moving Average and is visible into the Indicator List (Ctrl+I)

Thus added MS is visible on chart but how could I found its current value throu the script.

I know I could calculate it in the script, but I'd like to use the existing value because for some reasons user might change MA values (price, per, etc). in this case the script have to be re-compiled if MA is calculated in the code.

I hope the question is clear enough. Thank you

Ivo