Customizing Swagger Spec With An IOperationProcessor

AddCustomHeader.cs

internal sealed class AddCustomHeader : IOperationProcessor
{
    public bool Process(OperationProcessorContext context)
    {
        var hdrParameter = new OpenApiParameter()
        {
            Name = "x-custom",
            Kind = OpenApiParameterKind.Header,
            IsRequired = true,
            Type = JsonObjectType.String,
            Default = "xyz",
            Description = "The description of the field"
        };

        context.OperationDescription.Operation.Parameters.Add(hdrParameter);

        return true;
    }
}

Program.cs

builder.Services.SwaggerDocument(o =>
{
    o.DocumentSettings = s => s.OperationProcessors.Add(new AddCustomHeader());
});