While using SharePoint Designer I haven’t found it possible to set the placeholder values, therefore a workaround for this is to use JQuery.
To do this, go to https://code.jquery.com/ and get the latest CDN
Then copy the script below, modifying the relevant sections
| <script type=”text/javascript” src=”https://TENANT.sharepoint.com/sites/YourSharePointSiteCol/SiteAssets/CDN/jquery-3.3.1.min.js”></script> <script> $(document).ready(function(){ |