Sorry to present the antithesis to an existing answer once more: if the string is neither reusable nor public, a constant makes no sense in my book. (And yes, this question is opinion-based ;-))
I see why you use a string-variable inside the medthod (to keep it readable for longer strings) which I find totally OK.
In general: aim to write the code for the audience of developers who come after you. In my experience, this future developer will dig through your code with a debugger because something goes wrong. Thus, he will not read the class in its entirity because he's curious, he will jump into that specific method from a stacktrace. All information that is locally there will be beneficial, as there is less jumping around in the code.
