Custom activity charts

How can I access the best maximum pulses in the last 42 days? I don’t know if this is possible

Yes you can:

Example pulling the athletes MMP power curve for the last 42 days (from activity date) and extracting best 5m power:

let mmp42d = icu.bestPower['42d']
let best5mWatts = mmp42d.getWatts(5 * 60)

Sorry I just realised you were probably looking for heart rate data. You can do:

let hr42d = icu.bestHR['42d']
let best5mBpm = hr42d.getBpm(5 * 60)

Everything available is listed here.

Any chance to access fit file messages as in custom activity fields?