Q:

How to pass multiple string values from one page to another?

I figured out how to pass one value as a string from a text box from one page to another using a query string.
But I don't know how to pass multiple string values, for example I have a wrkTbx,rstTbx and roundTbx.
How do I pass all three values to another page?
This is my current implementation:
Workout page:
        private void appBarAddBtn_Click(object sender, EventArgs e)
        {
            NavigationService.Navigate(new Uri("/MainPage.xaml?key=" + wrkTbx.Text, UriKind.Relative));
        }

MainPage:
        protected override void OnNavigatedTo(NavigationEventArgs e)
        {
            base.OnNavigatedTo(e);
            if (NavigationContext.QueryString.ContainsKey("key"))
            {
                string val = NavigationContext.QueryString["key"];
                MessageBox.Show("value is:  " + val);
            }

        }

A:

You can separate multiple QueryString parameters with an ampersand (&). But you also need to be sure to properly encode those values, because of special characters they may contain (like an ampersand, for example).
Here's a modified version of your code, that passes all the values you mentioned.
Workout page:
private void appBarAddBtn_Click(object sender, EventArgs e)
{
    string url = "/MainPage.xaml" +
        "?wrkTbx=" + System.Net.WebUtility.UrlEncode(wrkTbx.Text) +
        "&rstTbx=" + System.Net.WebUtility.UrlEncode(rstTbx.Text) +
        "&roundTbx=" + System.Net.WebUtility.UrlEncode(roundTbx.Text);

    NavigationService.Navigate(new Uri(url, UriKind.Relative));
}

MainPage:
protected override void OnNavigatedTo(NavigationEventArgs e)
{
    base.OnNavigatedTo(e);

    if (NavigationContext.QueryString.ContainsKey("wrkTbx"))
    {
        string wrkTbx = NavigationContext.QueryString["wrkTbx"];
        MessageBox.Show("wrkTbx value is: " + wrkTbx);
    }

    if (NavigationContext.QueryString.ContainsKey("rstTbx"))
    {
        string rstTbx = NavigationContext.QueryString["rstTbx"];
        MessageBox.Show("rstTbx value is: " + rstTbx);
    }

    if (NavigationContext.QueryString.ContainsKey("roundTbx"))
    {
        string roundTbx = NavigationContext.QueryString["roundTbx"];
        MessageBox.Show("roundTbx value is: " + roundTbx);
    }

}

