Use findElement
instead of findElements
driver.findElement(By.xpath("//input[@id='invoice_supplier_id'])).sendKeys("your value");
OR
driver.findElement(By.id("invoice_supplier_id")).sendKeys("value", "your value");
OR using JavascriptExecutor
WebElement element = driver.findElement(By.xpath("enter the xpath here")); // you can use any locator
JavascriptExecutor jse = (JavascriptExecutor)driver;
jse.executeScript("arguments[0].value='enter the value here';", element);
OR
(JavascriptExecutor) driver.executeScript("document.evaluate(xpathExpresion, document, null, 9, null).singleNodeValue.innerHTML="+ DesiredText);
OR (in javascript)
driver.findElement(By.xpath("//input[@id='invoice_supplier_id'])).setAttribute("value", "your value")
Hope it will help you :)