Hi @Mary Ma,
I'm glad to hear you solve the problem ,if you have any issue about SharePoint, you are welcome to raise a ticket in this forum.
By the way, since the Microsoft Q&A community has a policy that "The question author cannot accept their own answer. They can only accept answers by others." and according to the scenario introduced here: Answering your own questions on Microsoft Q&A, I would make a brief summary of this thread:
[How to map custom field in sharepoint in indexer for azure cognitive search]
Issue Symptom:
when using Cognitive Search, the newly added fields "url" and "doc_title" respond as null, while other traditional fields (eg metadata_spo_item_path, content) have content.
Solution:
Add a space in additional column in datasource. i.e. if the sharepoint column name "Doc Title", then have to add it as additionalColumns=Doc Title.
You could click the "Accept Answer" button for this summary to close this thread, and this can make it easier for other community member's to see the useful information when reading this thread. Thanks for your understanding!
Best Regards
Cheng Feng