Solution found in https://youtu.be/kbqyQ4RJZlc from minute 5 onwards.
Basically to send form data into Plasmic CMS using API follow the steps below
[1] Create Plasmic CMS required
[2] Then create the Plasmic CMS in integration by referring to Plasmic CMS - API Reference | Learn Plasmic
[3] Use the form Interactions → On Submit to create the integration as detailed in the youtube link from minutes 5 onwards.