Are you referring to MoRef ID in vCenter / vCenter Object ID?
The NSX REST API guide suggests to use the MOB browser as described in this KB: VMware KB 1017126 - Looking up Managed Object Reference (MoRef) in vCenter Server
Other alternative ways such as using NSX Manager Central CLI and PowerCLI to find MoRef ID are described in these blog posts:Bayu Wibowo | vExpert NSX, VCIX6-DCV/NV, Cisco Champion, AWS-SAA
Author of VMware NSX Cookbook http://bit.ly/NSXCookbook
https://nz.linkedin.com/in/bayupw | twitter @bayupw