GetSiteScriptFromWeb source code

[vixxg]

Hi,
I successfully get site script from target website using Get-PnPSiteScriptFromWeb, and I want to get site script from the same website using C# but failed.
I look up the code in /src/Commands/SiteDesigns/GetSiteScriptFromWeb.cs , and found that pnp-powershell doing this by Tenant.GetSiteScriptFromSite(Url, tenantSiteScriptSerializationInfo);
When I try to write codes, I found that GetSiteScriptFromSite() is not a static method, maybe it's about the version of Microsoft.Online.SharePoint.TenantAdministration?
So I create a new object instance and write code like this:

Tenant tenant = new Tenant(ctx),;
var script = tenant.GetSiteScriptFromSite(Url, tenantSiteScriptSerializationInfo);
ctx.ExecuteQueryRetry();

and I receive error message "current site is not a tenant administration site" when executing ctx.ExecuteQueryRetry();
It looks like an error related to permissions, but I can use powershell to get the sitescript without problem.

Any advice would be appreciated.

environment: PnP.PowerShell 1.6.0