In a microflow, you can use the getGUID Java action from the Community Commons module to get the Mendix generated ID. Then if you want to store this for later access, you can create a column in your entity and use a Change action to update the attribute.
If you don't have the Community Commons module in your project, it can be downloaded from the appstore.
Perhaps you could share a bit about what you are trying to accomplish as using the Mendix generated GUID is rarely the best way to build your model in Mendix.