To effectively assist you, I want to confirm/reject the following:
Departing from the assumptions that I mention above, in combination with the information you have provided in your post. You should be able to configure the two parameters that are missing (AccessKeyID and SecretAccessKey) in your custom runtime settings (Sprintr → Your application → Environments → Your environment → Runtime → Custom Runtime Settings).
For information on Runtime Customization, please refer to Mendix Docs.
For information on how to retrieve these two values for your AWS account, please refer to AWS Identity and Access Management (User Guide).
I hope this helps, do feel free to reach out to me with additional inquiries.
I'm following up to see if your issue has been resolved or whether I could be of assistance.
I'm happy to help!