This is how we can set manual declaration of records using SharePoint server object model.
using MSFTREC = Microsoft.Office.RecordsManagement.RecordsRepository;
SPList result = web.Lists["ListName"];
MSFTREC.Records.ConfigureListForAutoDeclaration(result, false);
SPFolder rootFolder = result.RootFolder;
rootFolder.Properties["ecm_AllowManualDeclaration"] = bool.TrueString;
rootFolder.Properties["ecm_IPRListUseListSpecific"] = bool.TrueString;
rootFolder.Update();