You can create Azure key vault and store your key in it, then use "Get secret" action in your logic app to access the key you stored just now. After that, you can use the key from "Get secret" action in your rest api in logic. By the way, you may need to grant permission to your logic app to access the azure key vault. The page I provided shows the way of PowerShell and Azure CLI to grant the premission, you can also do it on azure portal(shown as below screenshots).
This helped me in understanding the role of ** Get secrets**, but again when the user is given the deployed logic app as a bundle(package) to use, I need a POI(point of Interaction) (Any Programmable Intervention like template in json or a UI for that matter) for the customer to use wherein he/she puts in the API keys,secret Keys etc of their own REST API , depending on whatever Authentication method is used. This approach certainly gives me a sense of clarity. Thankyou!