The API is (currently) replace-only, so you do need to get the current items first and append them (as you suspect).
That said, I've tried to make that a bit easier by providing some python client code that does the get+append behind the scenes. It is on GitHub for the Backstory API: https://github.com/chronicle/api-samples-python/blob/master/lists/append_to_list.py
...and there is also a v1alpha version of that for the new Chronicle REST API: https://github.com/chronicle/api-samples-python/blob/master/lists/v1alpha/patch_list.py
This blog describes the latter: https://www.googlecloudcommunity.com/gc/Community-Blog/Update-Reference-Lists-with-Python-and-the-new-Chronicle-REST/ba-p/717241
Hi DanDye, thanks you for the answer. I really appreciate it. I was playing with an old version api-sample-python. I need to update it.
Best regards,