Is it possible to get a future Amazon S3 link immediately when I start uploading the file?

Not in general. An Assembly can create several outputs, and later Steps can depend on the input’s contents or metadata. Transloadit cannot promise their final locations before processing finishes. Read the completed Assembly Status to obtain the actual export results.

For a workflow with one known output, your backend can choose a destination object key in advance and place that literal key in the export Robot’s path. Authenticate the user, authorize the destination and record the upload’s association in your application. Follow verifying an upload’s user; do not treat client-supplied fields as proof of ownership or let them choose an authorization-sensitive destination.

Protect the permitted instructions with Signature Authentication. When using a saved Template, follow the Template override guidance as well. A signed request does not make separately supplied multipart fields trustworthy.

A preselected object key is not a guarantee that processing or export will succeed, and it is not necessarily a publicly accessible URL. Bucket access policy, URL encoding and the destination configuration still matter. Do not expose an object as an authorized user’s result until your backend verifies the completed Assembly against its own upload record. Use unique keys to avoid overwriting another upload; see the S3 permission and overwrite guidance.