Hi,
To submit a full these via the REST API, you'd want to use the submission API docs at
https://github.com/DSpace/RestContract/blob/main/submission.md. There's no endpoint that only accepts a file/bitstream...you have to start a submission, add metadata and then upload the file all via the REST API.
Obviously, this can be easier to do via the DSpace User Interface. But, that's how you'd go about it if you need to perform this via the REST API.
Tim