Business / Domain Logic Help

Hi I'm wondering if anyone here has ever written Business / Domain logic for an application they developed? I have been tasked with writing some business logic for a platform and I cant find concrete examples online. I have designs ready and i guess I just need to explain how they would work in the backend? Imagine a simple Login screen with two inputs (Username, Password), Login Button (Submit), little text under the button 'Forget password?'. What would the domain logic be for this scenario?