how to update Navigation entity with Microsoft.PowerPlatform.Dataverse.Client.Dynamics

I am using Microsoft.PowerPlatform.Dataverse.Client.Dynamics package to update CRM entity.

to update a normal field,

entity.Attributes["ecg_proposal_url"] = sendURL ;
await client.UpdateAsync(entity);

works fine. but when I try the same with a navigation field (pointing to user) , I get error:

"CRM do not support direct update of Entity Reference properties, Use Navigation properties instead"
The search did not show any example. Also the DOCs are very slim.
How do I update it with field name "_ecg_proposal_written_by_value" ?
Thank you.
The code that causes the error is:

entity.Attributes["_ecg_proposal_written_by_value"] = writtenByID;

Or

entity.Attributes["ecg_proposal_written_by"] = writtenByID;

it should have a user ID

a lookup field needs to specify the table associated with it.
With the C# SDK you need to assign a EntityReference object.

Something like:

entity["ecg_proposal_written_by"] = new EntityReference("systemuser", writtenByID);

assuming the lookup points to the systemuser (User) table.

Note that with the C# SDK you need to use the logical name of the field (ecg_proposal_written_by) and not the name used with Web API / OData 4.0 syntax (in your case _ecg_proposal_written_by_value)