How do I execute code when a form opens in D365FO?

Extend the form's init() method when the logic is form-level.

[ExtensionOf(formStr(MyForm))]
final class MyForm_Extension
{
    public void init()
    {
        next init();

        // Your initialization logic.
    }
}

If what you are initializing belongs specifically to a datasource, put the logic on the datasource instead. Keeping logic close to what it affects makes the form a lot easier to maintain.