The NewDocSet.aspx Page
If you look at the URL of this page you'll see it looks something like
You can see that
NewDocSet.aspx lives in the LAYOUTS folder of SharePoint's "hive". This is a term that just refers to SharePoint's installation location on the server.
The hive on the server will be at
C:\Program Files\Common Files\microsoft shared\Web Server Extensions\15
And the LAYOUTS folder will be in the hive at
As you've already discovered, you can't edit
NewDocSet.aspx from the GUI. This is because this page sits in SharePoint's hive.
How to Customise NewDocSet.aspx
To make changes to this page, this file in the hive on the server is the one you need to edit. However, it is recommended you do not change the default file. Instead, make a copy of it first in the same location, call it
NewDocSet_custom.aspx, and change this file.
Once you've made your changes you need to tell SharePoint to point to this new file. To do this, open your site in SharePoint Designer. On the left click Lists and Libraries and open the document library that contains your Document Set content type. In the Content Types panel click your Document Set content type. In the Forms panel you'll see the location of the New Form for this content type. It will probably be currently set to
_layouts/15/NewDocSet.aspx. Change this to your new file, i.e.
_layouts/15/NewDocSet_custom.aspx. Click Save in SharePoint Designer.
Now when you attempt to create a new document set, it will navigate to your new page.
How to Auto Populate Name Field
NewDocSet_custom.aspx and find this line:
<asp:Content ContentPlaceHolderID="PlaceHolderMain" runat="server">
Directly under this line add this code:
$("input[id^='FileLeafRef']").val("ADMIN Do Not Edit");
Save it and proceed to create a new document set. You should now see the Name field auto populated.
NewDocSet_custom.aspx on the server. In this case this is what you would add in