Dynamic connection to Database ASP.NET - asp.net-core

Question

I want write autorization system in asp.net - user of Postgres (which created throws CREATE ROLE...) entering login and password and working with own tables. Problem is i working in ASP.NET core and connecting throws Dapper.
appsettings.json
{
"Logging": {
"IncludeScopes": false,
"LogLevel": {
"Default": "Debug",
"System": "Information",
"Microsoft": "Information"
}
},
"DBInfo": {
"Name": "coresample",
"ConnectionString": "User ID=username;Password=userpassword;Host=localhost;Port=5432;Database=Documents;Pooling=true;"
}
}
DocumentsRepository
public class DocumentsRepository : IRepository<Documents>
{
private string connectionString;
public DocumentsRepository(IConfiguration configuration, string login, string password)
{
connectionString = configuration.GetValue<string>("DBInfo:ConnectionString");
connectionString = connectionString.Replace("username", login);
connectionString = connectionString.Replace("userpassword", password);
}
internal IDbConnection Connection
{
get
{
return new NpgsqlConnection(connectionString);
}
}
public void Add(Documents item)
{
using (IDbConnection dbConnection = Connection)
{
dbConnection.Open();
dbConnection.Execute("SELECT addrecuserdocuments(#DocumentName,#Contents,#DocumentIntroNumber)", item);
}
}
public IEnumerable<Documents> FindAll()
{
using (IDbConnection dbConnection = Connection)
{
dbConnection.Open();
return dbConnection.Query<Documents>("SELECT * FROM selectdata()");
}
}
DocumentController
public class DocumentsController : Controller
{
private readonly DocumentsRepository dRepository;
public DocumentsController(IConfiguration configuration)
{
dRepository = new DocumentsRepository(configuration);
}
public IActionResult Index()
{
return View(dRepository.FindAll());
}
public IActionResult Create()
{
return View();
}
// POST: Documents/Create
[HttpPost]
public IActionResult Create(DocumentViewModel dvm)
{
Documents docs = new Documents { DocumentName = dvm.DocumentName, DocumentIntroNumber = dvm.DocumentIntroNumber };
if (ModelState.IsValid)
{
if (dvm.Contents != null)
{
byte[] imageData = null;
using (var binaryReader = new BinaryReader(dvm.Contents.OpenReadStream()))
{
imageData = binaryReader.ReadBytes((int)dvm.Contents.Length);
}
docs.Contents = imageData;
}
dRepository.Add(docs);
return RedirectToAction("Index");
}
return View(docs);
}
In Startup.cs i connection throws
services.AddSingleton(Configuration);
Please, help me understand, how i can call connection string and transfering login and password of user? And where? And is it possible?
Please, i realy need help with this!