Incase the element is within an iframe:

Then if you shift your focus to the Console tab you will find a dropdown with the first item being called top containing the desired frame as a child as follows:

Like wise select the desired iframe in the Console tab in which the desired element is located.
Next, given the HTML:
<input id="accountName" autocomplete="email" autofocus="" class="form-textbox-input fin-input__input" data-test="accountName-input" dir="auto" name="accountName" required="" type="text">
To populate the <input>
field with the text newValue you can execute the following command in the Console:
document.querySelector('input#accountName').value = "newValue"
Console snapshot:
