For me it works with all border points properly closed into a polygon. If the area is without intersecting elements, then a "space" (magic wand) -click with the fill tool selected does the job. If you have other elements intersecting the polygon then you should have the border elements selected while magic wand-clicking in/or on the border.
But the fill shape is not a complete match, as Archicad seems to converts the spline into arc segments.
| Archicad 4.55 - 26
| HP Z840 | 2× E5-2643 v4 | 64 GB RAM | Quadro M5000 | Windows 10 Pro x64
| HP Z4 G4 | W-2245 | 64 GB RAM | RTX A4000 | Windows 11