TipsTricksTests.java

@Test
public void setHiddenField() {
    //<input type="hidden" name="country" value="Bulgaria"/>

    var theHiddenElem   = driver.findElement(By.name("country"));
    JavascriptExecutor javascriptExecutor = (JavascriptExecutor) driver;
    javascriptExecutor.executeScript("arguments[0].value='Germany';", theHiddenElem);
    String hiddenFieldValue = theHiddenElem.getAttribute("value");

    Assert.assertEquals("Germany", hiddenFieldValue);
}