Internal key assignment; enter a reuse area

I'm creating a custom object with it's own data model to govern in MDG.

I want the object to be in flex mode and need a temporary key to be assigned so I can then automate a proper key at the approval stage.

However I am receiving the following message:

Internal key assignment; enter a reuse area

Message No. USMD1586

Diagnosis

A reuse active area must be specified for entity types with internal key assignments.

System Response

You cannot activate the data model.

Procedure

Change the type of key assignment, or enter a reuse active area for data model ** or entity type ******.

How can I assign temporary keys for a flex entity?