One way is to update the client_LocationBasedDefaults.html file.
Use a SP HTTP GET to read the file content and put the result into a string variable
Ex:
_api/Web/GetFileByServerRelativePath(decodedurl='/sites/yoursite/Shared Documents/Forms/client_LocationBasedDefaults.html')/$value?binaryStringResponseBody=true
Then you will need to build a string to add your folder defaultvalue info at the end
You new folder info that you will append to the file content will be something like:
<a href="/sites/YouSite/Shared%20Documents/yourNewFolder"><DefaultValue FieldName="udlProjectNo">YourValue</DefaultValue></a>
Note: If your folder contains spaces, replace spaces with %20'
Then write back the new file content with a SP HTTP PUT
_api/Web/GetFileByServerRelativePath(decodedurl='/sites/YourSite/Shared Documents/Forms/client_LocationBasedDefaults.html')/$value
The body will be your new default values string.
Microsoft should provide better api commands to do these things....
Hope it helps