To insert your Shipment Tracking - Unified API key on Postman, just make sure you have the parameter “DHL-API-Key” under Headers with your respective key as explained in the screenshot below. You don’t have to insert anything under Authorization.
You can find the API Keys of your apps when clicking on your profile icon after logging in to the DHL Developer Portal, then click on one of your apps, and finally click on "Show key". The API Key from one app will be good for all approved APIs you have added previously.
Not all APIs have an authorization method via API Key, please read carefully the instructions of other DHL APIs to know more about it.