Searched it on Google and couldn't find it. Consider the following literal statement:
If there exists a supremum M for A, where A is a set contained within the one dimensional continuum R, then M may or may not be a member of A.
I wish to write it as a formal statement.
I'm not sure if a notation question qualifies for a valid question on this exchange, so please alert me if it doesn't.
Since tertium non datur, the propsition $M\in A$ or $M\notin A$ means that A is a (decidable, i.e. well-defined) set. Nothing is added.
Instead you want to say there exist $A$ and $B\subset \mathbb R$ such that $\sup A\in A$ and $\sup B\notin B$. A specific example is $A = [0,1]$ and $B = (0,1)$ with $\sup A = \sup B = 1$.