You need to use the ::placeholder selector on your CSS. Unfortunately, you can only do this using a CSS file. I'm not sure Construct 3 can load them.
Just discovered that since r158 there is a LoadStylesheet action for the Browser object. So, just add the Browser object to your project, create a CSS stylesheet as project file and load it.
You can download this file and check how it's done.
Hope this helps...
Cheers!