How to get heart rate max from Athlete by strava api ?

There isn’t a direct way.

You could get the heart rate zones with /api/v3/athlete/zones, take the min heart rate for the highest zone there and use the formula * 100 div 0.97. That will only work though when the athlete didn’t set custom zones ("custom_zones":false in the result).

The other way would be getting every activity including details and determine the highest max_heartrate from them all. That of course is expensive and prone to measurement errors and the problem that the athlete maybe never got all out.