No appsettings.json:

{
  "ConnectionStrings": {
    "DefaultConnection": "Host=localhost;Port={seu_port};Database={seu_db};Username=postgres;Password={sua_pwd}"
  },

  "Logging": {
    "LogLevel": {
      "Default": "Information",
      "Microsoft.AspNetCore": "Warning"
    }
  },
  "AllowedHosts": "*"
}

No Program.cs:

var builder = WebApplication.CreateBuilder(args);

// Configura o DbContext para PostgreSQL, usando a string no appsettings.json
builder.Services.AddDbContext<ApplicationDBContext>(options => {
    options.UseNpgsql(builder.Configuration.GetConnectionString("DefaultConnection")); //busca nas configs no json
});

builder.Services.AddEndpointsApiExplorer();
builder.Services.AddSwaggerGen();

var app = builder.Build();

if (app.Environment.IsDevelopment())
{
    app.UseSwagger();
    app.UseSwaggerUI();
}

app.UseHttpsRedirection();

app.Run();

Executando Swagger

cd minha_api
dotnet watch run

• Lembrar de SEMPRE salvar o arquivo antes de executar ele para evitar erros chatos

TesteConexãoController