Create an isoparametric curve through a point

Hello everyone! there are a point on surface, How do I create an isoparametric curve from this point. Thanks!

https://github.com/mcneel/rhino-developer-samples/blob/master/cpp/SampleCommands/cmdSampleExtractIsoCurve.cpp This code creates an isoparametric curve by picking up a point on the surface.

If a point has been placed on the surface, how to create an isoparametric curve through this point.

huh? that isn’t what extractisocurve does?