string foo = InkBunnyUrls.Login + "&username=" + txtUsername.Text + "&password=" + txtPassword.Password;
//foo = https://inkbunny.net/api_login.php?output_mode=xml&username=test&password=foobar#1
string url = System.Net.HttpUtility.HtmlEncode(foo);
// url = https://inkbunny.net/api_login.php?output_mode=xml&username=test&password=foobar#1

string foo = InkBunnyUrls.Login + "&username=" + txtUsername.Text + "&password=" + txtPassword.Password;
        //foo = https://inkbunny.net/api_login.php?output_mode=xml&username=test&password=foobar#1
        string url = Uri.EscapeUriString(foo);
        //  https://inkbunny.net/api_login.php?output_mode=xml&username=test&password=foobar#1

string encoded = HttpUtility.UrlEncode("#");    // "%23"

string encoded = Uri.EscapeDataString("#");    // "%23"