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According to microsoft Do not use resource files to store passwords, security-sensitive information, or private data.

Is there a file that I can securely store passwords in? I am using a third party API that requires authentication through a email and password I have setup. I don't want to hardcode the values into the login function as that would be exposed if someone were de-compile the executable.

Is there a file I can write those details in, that C# projects would encrypt for me and I would be able to access programmatically?

You should never store any kind of credential information in a file. It's just as bad as storing them in a resource file. I suspect by file you mean a recommend storage for saving credentials. There's a Nuget library called Credential Management:

http://nuget.org/packages/CredentialManagement/

The library is just wrappers around Windows API calls for credential management. UWP applications SDK has the same library already built-in.

Here's some code on how to use the code

https://geoffhudik.com/tech/2017/09/14/console-app-credentials-and-the-windows-credential-manager/

I don't think you could embed such password to third party api directly into exe application or something that you give to users and feel safe. Even encrypted config file has to be decrypted at some point.

That said you generally create your own API on your own server and have middleware where you can have your secret keys only on your server. Because how are you going to change password if you need to change it? When you have your api middleware you can change that password and end users for your api will be able to login with whatever auth scheme you have for them. When this third party api will introduce breaking changes application that you deliver will break and you will have to update all installations, when you have middleware you can make fix in middle ware.