Securely storing a password locally that is used for web service

I have an application that authenticates against a third party web service by sending a username and password. At the moment I'm typing the password on a winform each time I start the application - but I need it to login automatically.

I'd like to store the username/password somewhat more securely than

Dim username as String = "username"
Dim password as String = "password"

I understand that I probably won't stop a determined hacker with access to my source code, but storing them as plaintext feels wrong.

I've found some similar questions on here but none that provide me with an answer I can use.

Edit: The web service isn't mine, it's an API that requires me to login to use.