Hi.
The link from @Jill_C is correct. As of today, there is no public API for submitting assets to Stock. The closest thing you can do to automate it is to have each Contributor share their SFTP credentials with your application, and you can use SFTP to upload the images. Once the files are on the SFTP site, they are automatically imported from a recurring job.
In general, Adobe Stock does not provide the API infrastructure for Contributor apps. There are no public APIs for getting sales data or reports of downloads. There has been discussion about creating this, but nothing has been implemented.
Thanks,
Christopher