Selenium Webdriver enter multiline text in form without submitting it

This is not a Selenium issue, pressing enter in a text field often submits the form. Usually you can bypass it by using Shift+Enter to insert a new line. Try this:

String myText = "first line\nsecond line";
myText = myText.replace("\n", Keys.chord(Keys.SHIFT, Keys.ENTER));
myElement.sendKeys(myText);

What worked for me using python 3 was make use of ActionChain as Tamas said and @Arount posted on Python and Selenium - Avoid submit form when send_keys() with newline

from selenium import webdriver
from selenium.webdriver.common.keys import Keys
from selenium.webdriver.common.action_chains import ActionChains

driver = webdriver.Chrome()
driver.get('http://foo.bar')

inputtext = 'foo\nbar'
elem = driver.find_element_by_tag_name('div')
for part in inputtext.split('\n'):
    elem.send_keys(part)
    ActionChains(driver).key_down(Keys.SHIFT).key_down(Keys.ENTER).key_up(Keys.SHIFT).key_up(Keys.ENTER).perform()