It’s not really clear what that long line is about, but anyhow, I’d suggest placing the button absolutely, in relation to the form element, which would be set to position: relative.
As Ralph said the question is not clear and does depend on many variables (such as where is the line coming from and how long is it or is it the fieldset border etc…).
Here is a starting point for the method Ralph proposed.
I have eschewed using the fieldset as the stacking context in favour of a div wrapper because fieldsets are special elements and are best left alone as far as stacking contexts are concerned.