public class HeaderParameter : IOperationFilter
{
    public void Apply(OpenApiOperation operation, OperationFilterContext context)
    {
        if (operation.Parameters == null)
            operation.Parameters = new List<OpenApiParameter>();

        operation.Parameters.Add(new OpenApiParameter
        {
            Name = "Custom-Header",
            In = ParameterLocation.Header,
            Required = false,
            Schema = new OpenApiSchema
            {
                Type = "integer",
                Default = new OpenApiInteger(120)
            }
        });
    }
}