How to embed or reference an existing schema?

[Plasmadog]

I have an API project that uses FastEndpoints, which uses NSwag for OpenAPI schema generation. I have one DTO type for which I already have a JSON schema, and I would like to just plug that in instead of allowing the schema to be inferred automatically.
The schema I have looks like this:

        {
            "$schema": "https://json-schema.org/draft/2019-09/schema#",
            "title": "Filter Expression Schema",
            "$ref": "#/definitions/expression",
            "definitions": {
                "expression": {
                    "type": "object",
                    // etc...
                }
            }
        }

The expression type includes some recursive refs to itself.

I do not know how NSwag and FastEndpoints work together, so I'm not sure how much of this is even related directly to NSwag, but this is what I have tried:

builder.Services.SwaggerDocument(o =>
{
    o.DocumentSettings = docSettings =>
    {
        docSettings.SchemaSettings.TypeMappers.Add(new ObjectTypeMapper(typeof(FilterExpression), JsonSchema.FromJsonAsync(mySchema, CancellationToken.None).Result));
    };
});

The problem is that I get an exception saying InvalidOperationException: Could not resolve the path '#/definitions/expression'. Presumably, the definitions part of my document is not being copied into the spec, but I very much out of my depth here. What can I do to ensure that my existing schema is fully incorporated into the spec as if the DTO had just been analyzed?

[Plasmadog]

So it looks like this is actually due to a bug in NJsonSchema, first reported back in 2018. It's the call to JsonSchema.FromJsonAsync that is throwing the exception.