To find your workspaceId, there are two main approaches you can use.
One is by signing on to your mParticle platform and navigating to the name of your current workspace in the top-left corner of the dashboard to open the workspaces menu. From here you can switch into any of your current workspaces, or click Settings to open the Workspace Settings page. You will then see a list of all workspaces associated with your accounts and the workspaceId for each. If you go to our guide here, it has a good visual on how to navigate to the workspace setting.
Another way is to make a GET request to the platform API and get a response containing a list of all workspaces and relevant information. Our documentation has a sample request of the request here listed under the REST Resources section and into the Workspaces subsection, all the way at the bottom.