Fill forms using selenium or requests

You cannot get access to Password field because it's not present on main page. To handle Password field you have to click Login button to get to Login page. Also you need to switch to iframe which contains authentication form

from selenium.webdriver.support.ui import WebDriverWait as wait
from selenium.webdriver.support import expected_conditions as EC

driver.get("https://www.santandertotta.pt/pt_PT/Particulares.html")
driver.find_element_by_xpath("//input[@title='Login de Particulares']").click()
wait(driver, 10).until(EC.frame_to_be_available_and_switch_to_it("ws"))
user = driver.find_element_by_name("identificacionUsuario")
user.send_keys("user")
pas = driver.find_element_by_name("claveConsultiva")
pas.send_keys("password")
pas.submit()

Once you access the url https://www.santandertotta.pt/pt_PT/Particulares.html first you have to click on the element with text as Login then only the the Nome and Password field appears but to access those fileds you have to switch to the frame with id as ws inducing WebDriverWait. Next to locate the element of Nome you have to induce WebDriverWait again as follows :

from selenium import webdriver
from selenium.webdriver.common.by import By
from selenium.webdriver.support.ui import WebDriverWait
from selenium.webdriver.support import expected_conditions as EC

driver=webdriver.Firefox(executable_path=r'C:\Utility\BrowserDrivers\geckodriver.exe')
driver.get("https://www.santandertotta.pt/pt_PT/Particulares.html")
driver.find_element_by_xpath("//input[@class='ttAH_button03']").click()
WebDriverWait(driver, 20).until(EC.frame_to_be_available_and_switch_to_it((By.ID, "ws")))
WebDriverWait(driver, 20).until(EC.element_to_be_clickable((By.XPATH, "//input[@class='inputlong' and @id='identificacionUsuario']"))).send_keys("your_name")
driver.find_element_by_xpath("//input[@id='claveConsultiva' and @name='claveConsultiva']").send_keys("your_password")
driver.find_element_by_link_text("Entrar no NetBanco Particulares").click()

Here you can find a relevant discussion on Ways to deal with #document under iframe