Hi,
Although the Fill Tool has a lasso mode (called Freehand), for OpenToonz to be able to perform a fill there must be a closed area previously defined by lines (i.e. drawn with the Brush or Geometric tools). So, you could define a closed area using any type of line (even a transparent one) and then you will be able to fill it.
Also, the Fill tool will always fill the entire closed area(s) enclosed by the lasso.
The only way to manually fill a certain region of the drawing is by using the Paint Brush Tool, but this tool is just like a brush that draws fills instead of lines.
(In OpenToonz a pixel color is either defined to be a "line" or an "area" (fill) color, so the program can then treat them independently while using effects, etc.)
It could certainly be handy if the Pain Brush Tool would have a Freehand (lasso) mode, indeed.