Conditional field in a complex component

I have a complex component that I use on 8 forms. On only one of them I want to have an extra text field. How can I do this?

You need to add a new input parameter to the complex component that will control the visibility of the textfield.
Then on the form add a new variable e.g. showTextfield and set its default value to true. Attach the variable to the complex component.
Add a visibility condition on the textfield such as getValue("showTextfield") == "true".
Remember to add listening on that variable.