I found the answer. Credit to @Tedinoz for helping me and for finding this link Google Apps Script to open a URL.
In this link 'TheMaster' provided an excellent answer. Two things are working here, an HTML source and a javascript. Within the HTML is var winRef = window.open(url1);
which opens the URL in another tab, however a dialog box opens.
The next line shows
winRef ? google.script.host.close() : window.alert('Allow popup to redirect you to '+url1) ;
What this seems to do is closes the dialog box within google sheet, which appears to be this section
winRef ? google.script.host.close()
So in short, when setting this up name the HTML file whatever you want, with the Javascript, make sure line 6 on the javascript reflects the same name as the HTML source.
HtmlService.createHtmlOutputFromFile('openUrl').setHeight(50), // 'openUrl is the name of the HTML source, name this whatever.
Once done, go to your button, attach the script which you name it by the javascript function modalUrl
.
Here is the full script. If 'TheMaster' ever sees this, please let me know if I nailed what I said above.
openUrl.html
<!DOCTYPE html>
<html>
<head>
<base target="_blank">
<script>
var url1 ='https://stackoverflow.com/a/54675103';
var winRef = window.open(url1);
winRef ? google.script.host.close() : window.alert('Allow popup to redirect you to '+url1) ;
window.onload=function(){document.getElementById('url').href = url1;}
</script>
</head>
<body>
Kindly allow pop ups</br>
Or <a id='url'>Click here </a>to continue!!!
</body>
</html>
code.gs
function modalUrl(){
SpreadsheetApp.getUi()
.showModalDialog(
HtmlService.createHtmlOutputFromFile('openUrl').setHeight(50),
'Opening StackOverflow'
)
}