I don't know your enterprise setup, but it could be that the AWS firewall is blocking the outbound request too. Just whitelisting the AWS Databricks ranges in the application you're posting to might not be enough. 

Edit: actually no that can't be the case because a 403 is remote server error. 
You might have only whitelisted the control plane IPs and not the compute plane (cluster/VM ip's)

Which IP's did you use?