MACD Histogram Label

The code required to do this has already been published but it was designed for a custom watchlist column, which is far more useful than applying this as a chart label. Here is the post to the solution that works as a custom watchlist column:

https://www.hahn-tech.com/ans/macd-watchlist-background-color/

If you truly want to adapt that for a chart study you only need to change the last two lines of code. Here they are in their original form for the watchlist column:

plot diff = Value - avg;
AssignBackgroundColor(if diff >= 0 then if diff > diff[1] then
Color.GREEN else Color.DARK_GREEN else if diff < diff[1] then
Color.RED else Color.DARK_RED);

And here are the changes you make to those two lines in order to get it to work as a label in a chart study:

def diff = Value - avg;
AddLabel(yes, "MACD", if diff >= 0 then if diff > diff[1] then
Color.GREEN else Color.DARK_GREEN else if diff < diff[1] then
Color.RED else Color.DARK_RED);

Just to be clear, you copy the entire code from the previous post I linked at the top of this answer, then replace the last to lines just as I have described above.