Modify the login.html in your theme directory.
if there is only a zip file (with you theme name) then extract that zip file in this same directory and set in the modeler in the settings your theme to Default. it will use the files in the theme directory then and also your modified login.html ;-) (don't edit the login.html in your deployment directory, it will be overwritteren every new deployment)
option 2 is to handle the login within the model with this widget: https://appstore.home.mendix.com/link/app/87/Mendix/Login-form you have to enable anonymous users for this in the security settings.