public class Startup
    {
     public void ConfigureServices(IServiceCollection services)
        {
            // ...
            services.AddControllers().AddJsonOptions(options =>
            {
                    options.JsonSerializerOptions.PropertyNamingPolicy = new SnakeCasePropertyNamingPolicy();
            });
            
            services.AddSwaggerGen(c =>
            { 
                c.SwaggerDoc("v1", new OpenApiInfo { Title = "My API", Version = "v1"});
                c.SchemaFilter<SnakeCaseSchemaFilter>();
            });        
        }
    }
    public class SnakeCasePropertyNamingPolicy : JsonNamingPolicy
    {
        public override string ConvertName(string name)
        {
            return name.ToSnakeCase();
        }
    }

    public static class StringExtensions
    {
        public static string ToSnakeCase(this string str)
        {
            return string.Concat(str.Select((character, index) =>
                    index > 0 && char.IsUpper(character)
                        ? "_" + character
                        : character.ToString()))
                .ToLower();
        }
    }
    
    public class SnakeCaseSchemaFilter : ISchemaFilter
    {
        public void Apply(OpenApiSchema schema, SchemaFilterContext context)
        {
            if (schema.Properties == null) return;
            if (schema.Properties.Count == 0) return;

            var keys = schema.Properties.Keys;
            var newProperties = new Dictionary<string, OpenApiSchema>();
            foreach (var key in keys)
            {
                newProperties[key.ToSnakeCase()] = schema.Properties[key];
            }

            schema.Properties = newProperties;
        }
    }