How to obtain JS values using C#

Basically, I need the data inside "window["_securedTouchToken"] = [data] ", but since the website is coded in javascript and my code is in C#, I have no clue what to do. I'm currently using RestSharp as well as NewtonsoftJson too.

My Code:

try
{
    var clienttest = new RestClient("https://static.securedtouch.com/sdk/pong.js?body=eyJwaW5nVmVyc2lvbiI6IjEuMi4wcCIsImFwcElkIjoid2lzaCIsImFwcFNlc3Npb25JZCI6IjJhYTRhMTg5OWVhZmEzZjQxNTY1MTFmODc3Njk4ZTI0MDI0ODA5MjQzY2E5YTdlMTU3NDI2M2QxZmUzMTM5ZWQiLCJkZXZpY2VJZCI6IklkLWY5Y2UzOTRlLThkNjMtNGE5OS1hNDM3LTVlMjIzYjRmMmMxYSIsImRldmljZVR5cGUiOiJHZW55bW90aW9uLUFuZHJvaWQtR29vZ2xlIE5leHVzIDVYIiwiY2xpZW50VmVyc2lvbiI6IjMuNC4wYSIsImFwcFZlcnNpb24iOiI0LjM4LjAgKDEyNDgpIn0%3D");
    clienttest.Proxy = new WebProxy(proxy.Proxy);

    var requesttest = new RestRequest(Method.GET);

    requesttest.AddHeader("User-Agent", "Mozilla/5.0 (Windows NT 10.0; WOW64; Trident/7.0; rv:11.0) like Gecko");
    requesttest.AddHeader("Pragma", "no-cache");
    requesttest.AddHeader("Accept", "*/*");

    IRestResponse responsetest = clienttest.Execute(requesttest);
    var contenttest = responsetest.Content;

    Console.WriteLine(contenttest);
}
catch (WebException)
{

}

Response:

(function () {
    window["_securedTouchToken"] = 'eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJkYXRhIjoiYVkwVDRpLzN1MFUzcVZiMkllZVJDVEtuN0hrOGdjeHJlbEFheUdTRTB4dStQQ2VZK2Jqb09rT0lndDVaUUJvVmdJRnM0NmFnWk1FS3MrMVdsVjRwZW4xamNmVENnakt6VXZxazg5a1p1Uk8zS0JBcnRWd1U3Tmd2bzdvTk15ekVuYXBJT0haMkxzZlN5RHQ2OWJQVEdWekxiVHZrRVJnakM0azBkN0xnNFRZWUJoUzNRSTBmcGtwWjk5MkUxU3UwUnZsNVNES0lBSUhKMElYeVh6TEJzajlxQUJoZWo0NzdkZVNPMnZUWVhsd0lSVjYxTzZOUTUrQnd0aXVPQkc5bE5KL2hCSzlOUFVVRjFnUm8zMm85Z0E9PSIsImlhdCI6MTU4NTc2MTM1MCwiZXhwIjoxNTg1ODQ3NzUwfQ.tnrvmyniVy9gtG-n1PPGUx3bYU1gonYM2qID2l8x1ak';
    document.dispatchEvent(new CustomEvent('SecuredTouchTokenReadyEvent'));
})();

Solution

Actually, I figured a way out later that today. If anyone has a similar problem to this, here's an example code

String St = "super exemple of string key : text I want to keep - end of my string";

int pFrom = St.IndexOf("key : ") + "key : ".Length;
int pTo = St.LastIndexOf(" - ");

String result = St.Substring(pFrom, pTo - pFrom);