If you want a sample you could refer to this tutorial: Azure Key Vault client library for .NET. And below is my test code.
string clientSecret = "client secret";
string clientId = "client id";
var secreturi = "https://****.vault.azure.net";
KeyVaultClient kvClient = new KeyVaultClient(async (authority, resource, scope) =>
{
var adCredential = new ClientCredential(clientId, clientSecret);
var authenticationContext = new AuthenticationContext(authority, null);
return (await authenticationContext.AcquireTokenAsync(resource, adCredential)).AccessToken;
});
var keyvaultSecret = await kvClient.GetSecretAsync($"{secreturi}", "testsecret").ConfigureAwait(false);
Console.Write(keyvaultSecret.Value);
