Auto populating the existing custom field in forms / Survey using custom HTML/Javscript and later saved in submisison is simple and straightforward. 
Retrieve the Custom Field ID:
   - Go to the Preview of the form.
   - Right-click on the page and select 'Inspect.'
   - Select the mouse pointer tool.
   - Click on the "Custom Field."
   - Copy the ID from the name and ID properties.
  
 Example: 
If you created a custom field named `xxTrustedFormCertUrl`, follow the above steps to get its ID.


Note:  myData  is just for example. In the customer custom HTML/Javascript code. Customer have to figure it out where is the captured data and replace the myData  
document.getElementsByName('customFieldId')[0].value = myData;    
document.getElementsByName('customFieldId')[0].dispatchEvent(new Event("input"));