How to make the FR955 automatically ask to set the elevation before starting an activity ?

Go to altimeter settings -> Auto Cal -> switch off

Or you can just add the altimeter or ABC screen as a glance or a control and just calibrate from there right before you start an activity.

Thank you for the response.
With Auto Cal, does my watch recalibrate its barometer during the activity using GPS location and DEM *during* the activity? If so, I will use the second method, because the barometer undergoes variations not necessarily due to changes in altitude (weather, temperature, wind...).

Garmin's documentation is frustratingly vague on this point (what a surprise), but as far as I can tell altimeter auto-calibration only makes a difference at the start of an activity. Once you start recording it looks like changes in elevation are based solely on barometric readings.

For example, here's an activity that I recorded a few weeks ago: https://connect.garmin.com/modern/activity/14011715805. If you download the GPX track and re-upload it to a site such a Caltopo that does its own DEM lookup then you'll see the elevation at the start was accurate to within 10', but after ascending 1,500' it was off by about 150'. (I also checked it using Strava's elevation correction feature, with very similar results). That's consistent with the sort of error you'd get with a barometric altimeter.

I think martinleroux is correct - AutoCal just works at the start of an activity. You can still re-calibrate the elevation manually during an activity by going to the controls menu (holding Light button) and selecting Altimeter (if its one of your controls). However, if you re-calibrate during an activity, it wont correct any elevation gain issues and it will only change the actual elevation value. For example, if the altimeter drifts upward during an activity and results in false elevation gain, re-calibrating during an activity will just bring the actual elevation back to the correct value, but the false elevation gain (total ascent) will not be subtracted out.

The elevation drift seems to be another issue with this watch that Garmin hasnt addressed yet:

https://forums.garmin.com/sports-fitness/running-multisport/f/forerunner-955-series/352892/altimeter-and-elevation-issues-still-occurring-calibration-temperature

https://forums.garmin.com/sports-fitness/running-multisport/f/forerunner-955-series/334732/altimeter-inaccuracy/

For my 955, colder temperatures consistently affect the altimeter more than expected (and compared to other watches). Garmin claims you just need to clean the watch, but that didnt help in my case. Just a heads up in case you have any issues.