I was able to get autofill of the users O365 username working by customizing the sign-in page. Essentially just capture the username value in the html [head] section and feed that value to the 'username' config property of the sign-in widget if it's not null.

Javascript for the header:

function getParameterByName(name, url) {

  if (!url) url = window.location.href;

  name = name.replace(/[\[\]]/g, '\\$&');

  var regex = new RegExp('[?&]' + name + '(=([^&#]*)|&|#|$)'),

    results = regex.exec(url);

  if (!results) return null;

  if (!results[2]) return '';

  return decodeURIComponent(results[2].replace(/\+/g, ' '));

}

const queryUrl = decodeURIComponent(window.location.href);

const redirect_uname = getParameterByName('username', queryUrl);

Addition to the sign-in widget script:

 // "config" object contains default widget configuration

    // with any custom overrides defined in your admin settings.

    var config = OktaUtil.getSignInWidgetConfig();

if(redirect_uname != null) {

config.username = redirect_uname;

    }