Not very efficient, but this should work:
protected void isPrimeButton_Click(object sender, EventArgs e)
{
int TestNumber = int.Parse(primeNumberTextBox.Text);
bool isPrime = true;
for (int i = 2; i < TestNumber; i++)
{
if (TestNumber % i == 0)
{
isPrime = false;
break;
}
}
if (isPrime)
yesNoPrimeTextBox.Text = "prime";
else
yesNoPrimeTextBox.Text = "not prime";
}