You need to generate a signed URL to upload your file in the bucket.

You can follow the instructions from https://aps.autodesk.com/en/docs/data/v2/tutorials/upload-file/#step-4-generate-a-signed-s3-url

You can also find a .NET example using our SDK at https://github.com/autodesk-platform-services/aps-sdk-net/blob/main/samples/oss.cs#L41-L53

For the Model Derivative part (conversion to IFC), you can use the Model Derivative SDK, as done at https://github.com/autodesk-platform-services/aps-sdk-net/blob/main/samples/modelderivative.cs#L31-L93

You just need to change the payload according to the RVT->IFC options