How can I create a Nuget Package whose source code is not visible?

I wrote a netstandard2.0 class library. I want to publish this as a Nuget package. I right-clicked on the project and made the necessary adjustments in the Package field. Likewise, when I right click and select Pack, it creates a nuget package. No problems so far.

When using the Nuget package I shared in another project, the entire source code of the library is visible. How can I prevent this? I want the user to see only the function names in the class. He should not be able to see the contents of the functions.

Thanks for your help.