Going above and below the zero line is the equivalent of going from positive to negative and vice versa. So the current MACD indicator is doing exactly what you're looking for.
Go into the indicator's settings and set Show breakout signals from No to Yes.
Check this version https://usethinkscript.com/threads/multi-timeframe-mtf-macd-indicator-for-thinkorswim.474/
Go into the indicator's settings and set Show breakout signals from No to Yes.
If it can also show macd crosses above/below zero for three higher timeframes, that would be great, e.g. when looking at the 30M, seeing arrows from the 1hr, 2hr, & 4hr crosses. The ability to select from a dropdown which four timeframes to plot would be awesome.
Check this version https://usethinkscript.com/threads/multi-timeframe-mtf-macd-indicator-for-thinkorswim.474/