Area filling like Paper.io using fllod fill algorithm

Hi all, am trying to implement paper.io logic. I know the filling can be done by using the flood fill algorithm. the problem that I am facing is flood fill algorithm takes the parameter, point or (x, y) that where it will start filling. how can I get that point value? I mean when the player made a closed loop I need to pass a point, that is inside of that loop. how can I get that point?

thanks for reading it.