Problem With Adding Script To Seattle.Html

Hello:

I was told to add the following script to target WIKI PAGE libaries only. I added the code to my seattle.html but when i open a page in that wiki library its not showing the the page name. Any idea how to fix this? or is there an alternate way to do this?

<scripttype="text/javascript"src="https://code.jquery.com/jquery-3.4.1.js"></script><scripttype="text/javascript">
						$( document ).ready(function(){if(window.location.href.indexOf("MyLibrary")>-1){
						     $("#PageName").html(decodeURI(window.location.href.split('/')[window.location.href.split('/').length-1].split('.')[0]));}});</script><divid="PageName"></div>

Someone said:

You can add into Master Page so it will apply to all wikipages in the library, open seattle.html in SharePoint Designer, add script under"mainContent" tag: ( i did this but it did not work for me)

nk