Browser signature authentication using Uppy + Transloadit

We’re looking into Uppy integration, and I can’t find clear documentation on how to ping our back-end for signature generation from a browser front-end. We need to restrict uploads to logged-in users, so prior to initiating upload we need to hit our back-end, verify the user is logged in, then generate the transloadit signature for signature Authentication.

Can someone point me at an example of this flow? I assume it’s a common one. Or, if I have this wrong, could you suggest the best way for us to incorporate login verification prior to Uppy>Transload>S3 flow?