Add the following three commands to a script and assign the script to a button:
1) A Send Event.
Clear the “Bring target app to foreground” and enter in Calculation. The calculation from the example has a folder + the new folder filename from a field in FMP
"cmd /c md " & Quote ( "C:\folder\Another Folder With Spaces\Last Folder Needs Double Slash\\") & TableName::FieldName
2) Pause/Resume Script
For duration: 0.1 seconds
3) Open URL
Same as the one you just created, but formatted for url
"file://C:/folder/Another Folder With Spaces/Last Folder Just Single Slash/" & TableName::FieldName
The button will now create the new folder and open it or if it already exist, it will not delete or alter it, but just open it.
All spaces, quotes and slashes are important. Make sure you don't forget anything, or just copy&paste ;)

