What you need is either
from NL.QuantumATK.ConfigurationConverters.CIFConverters import configurationToCIF
with open("myfile.cif", 'w') as f:
f.write(configurationToCIF(configuration))
or
from AddOns.VASPPlugins.IO import writePOSCAR
writePOSCAR(configuration, "POSCAR")
NOTE: In case someone wants to use this export function for more advanced crystallography purposes, bear in mind that the QuantumATK CIF exporter is rather primitive and can only save structure as P1, so it will not use the correct spacegroup symmetries to reduce the atom list to the irreducible basis.