1. Add a new Import at "New Import", upload a random.txt (json, zip etcs) file.
2. Continue to "Drag & Drop" step. Put the XSS payload in the title form, e.g: laladee"><img src=a onerror=alert('title')>
or put XSS payload in Unique Identifier form at "Import Settings" step.
3. Continue to the end of steps and run the Import (which will trigger the XSS)
4. The XSS will be triggered again when going to Manage Imports > Run Import, Manage Imports > Run Import > Confirm & Run the import, Manage Imports > Edit Import > Preview

Note: v3.6.2 fixed the XSS in when running the import, but not in the preview