Create a custom local coordinate system

[BerendVeldkamp]

I am developing an add-in in .NET, and have a use case where a featureclass has a local coordinate system, unrelated to any real location on earth.

When I open a map that contains this data everything works, but ArcGIS Pro gives a warning about an unknown spatial reference, and I'd like to get rid of that message, by providing a fictional spatial reference.

I tried creating the featureclass with a spatial reference (0), but that makes no difference: 

SpatialReferenceBuilder.CreateSpatialReference(0);

I also tried creating a spatial reference using a GP tool:

var gpResult = await Geoprocessing.ExecuteToolAsync("management.CreateSpatialReference", []);

This returns a GPSpatialReference object, with this value:  

{B286C06B-0879-11D2-AACA-00C04FA33C20};-450359962737.049 -450359962737.049 10000;-100000 10000;-100000 10000;0.001;0.001;0.001;IsHighPrecision

But I don't know how to convert this to a .NET SpatialReference object.

Any ideas on how to proceed?