// In Microsoft.OpenApi project
public partial class OpenApiFormat {
    private string _format;

    public static OpenApiFormat Json { get; } = new OpenApiFormat("json");

    public OpenApiFormat(string format)
    {
        _format = format.ToLower();
    }

    // Implicit conversion from OpenApiFormat to string
    public static implicit operator string(OpenApiFormat format) => format._format;

    // Implicit conversion from string to OpenApiFormat
    public static implicit operator OpenApiFormat(string format) => new OpenApiFormat(format);

}

// In Microsoft.OpenApi.Yaml project
public partial class OpenApiFormat {
    public static OpenApiFormat Yaml { get; } = new OpenApiFormat("yaml");
}

var readResult = await OpenApiDocument.LoadAsync(streamOpenApiDoc,OpenApiFormat.Json);

var readResult = OpenApiDocument.Load(new StringReader(stringOpenApiDoc), OpenApiFormat.Yaml);

var readResult = OpenApiDocument.Parse(stringOpenApiDoc, OpenApiFormat.Yaml);