There is no suggested method; if you need protection
consult an attorney. Copyright law is exceedingly complicated. In the US, in other countries the basis is different.
In the About field, you can post a copyright notice. An obvious copyright notice might increase your standing in a lawsuit.
You can post a licensing notice in the app. A notice explains your licensing terms. Enforcing is another matter.
This post is NOT legal advice. There are links where you can look for more information to decide what you want to do. Consult an attorney if you believe you need to protect something very valuable.