cannot get ' impressions Share' metric from API

No idea, I have asked the same question on here as well as through support and an Apple rep and have not received or found a solution. My assumption is the endpoints in the public API mimic the internal API endpoints that are called when you are viewing metrics for in the management portion of ASA and do not mimic the reporting endpoints in the custom reports section. I am guessing this is why there are so many different reporting endpoints in the public API and there is only one endpoint (https://app.searchads.apple.com/cm/api/v1/customreports/...) called in the internal API when you run a custom report in ASA. They need to build a proper reporting API endpoint that mimics the custom report functionality in ASA.