It's a shame, I didn't find a workable solution.
And as usual, Withings support is not responding. 
Nevertheless, thank you for all your efforts, dear @JustinL!
BTW, I've found a completely different solution to my problem:
I use the "Google Fit" integration of Home Assistant to get the current Steps of today; And thanx to HADB I can finally use this value also in Hubitat. 
