The place holder text is the text that is seen in the username and password textbox on the login page prior to entering a username and password.
The default placeholder text is:
"User Name (e.g., domain\\name)"
From the Administration console change the placeholder text under Settings / Language Settings by selecting the login_en.json and changing the following:
"UsernamePlaceholder": "User Name (e.g., domain\\name)"
For example the following will change the placeholder texxt to "temp" for both.
Save the login_en.json file and upload it with the Browse button on the same page.
Note that at this time it is not currently possible to change the placeholder value for the login form per Front End Authenticator.