Please note that in the web console and Support documentation, Tabs are alternatively known as 'Tags,' 'Fields,' or 'Document Custom Fields.' Additionally, the values within Tabs are referred to as 'form data.'

https://www.docusign.com/blog/dsdev-trenches-tabs-and-custom-fields#:~:text=Note%20that%20Tabs%20are%20referred,enter%20data%20of%20their%20own.

Is it possible to create Custom Document Fields via the API? yes it is possible , please refer https://developers.docusign.com/docs/esign-rest-api/reference/customtabs/

Is it possible to place them on the Document before it is sent to Docusign? This can be achieved  https://support.docusign.com/s/document-item?language=en_US&rsc_301&bundleId=gbo1643332197980&topicId=nwo1611173513994.html&_LANG=enus through API https://developers.docusign.com/docs/esign-rest-api/esign101/concepts/tabs/prefilled-tabs/

Is it possible to retrieve the populated Custom Document Field Values that have been filled out by the client via the API? To download form data via API, you can use GET /v2/accounts/{accountId}/envelopes/{envelopeId}/form_data https://developers.docusign.com/docs/esign-rest-api/reference/envelopes/documentgeneration/getenvelopedocgenformfields/

Thanks & Regards

Sai HariPriya Dandamudi | Developer Support Engineer | DocuSign