assets/temp/Program.cs

using Azure.Deployments.Expression.Expressions;
using Newtonsoft.Json.Linq;
 
var funcs = new ExpressionBuiltInFunctions();
 
var parameters = new FunctionArgument[]
{
    new FunctionArgument(new JValue("subscription")),
    new FunctionArgument(new JValue("resourcegroup")),
    new FunctionArgument(new JValue("managedapp")),
 
};
 
var prefix = funcs.EvaluateFunction("uniqueString", parameters, new ExpressionEvaluationContext()).ToString();
Console.WriteLine(prefix);