custom-google-slides-font-dropdown.js

function onOpen(e) {
  SlidesApp.getUi()
    .createMenu("Custom Fonts")
    .addItem("Georgia", "formatText")
    .addToUi();
}

function formatText() {
  var selection = SlidesApp.getActivePresentation().getSelection();
  if (selection) {
    var selectionType = selection.getSelectionType();
    if (selectionType == SlidesApp.SelectionType.TEXT) {
      var slide = SlidesApp.getActivePresentation().getSlides()[0];
      var shape = slide.getPageElements()[0].asShape();
      var textRange = shape.getText();
      textRange.getTextStyle().setFontFamily("Georgia");
    }
  }
}