How to use Source in ElasticSearch Net

Elastic Stack Elasticsearch
1

How can I project fields in the Elasticsearch Net 8?

I tried:

SourceFilter filter = new SourceFilter();

filter.Includes = new List<Field>() ..  hm not the way

SourceConfig sourceConfig = new SourceConfig(filter);

SearchResponse<PostDocument> searchResponse = await client.SearchAsync<PostDocument>(s => s
.Index(index_name)
.From(page)
.Size(countPerPage)
.Source(sourceConfig)
.Query(..)

Try 2

.Source(null).Fields(
 fs => fs.Field(f => f.Id),
 fs => fs.Field(f => f.PostId),
 fs => fs.Field(f => f.Title))

I also saw SourceIncludes which takes in Fields but its not an expression same as the chain Fields

Any help? docs is still very limited