Hi Glenn,
Good news, you don't actually need custom fields for this.
Devolutions password manager's generic import maps columns by their header name straight to the native fields. So instead of routing URL into Custom #1, just rename your CSV header row to use recognized names: Name, Username, Password, URL, Notes, Folder. The URL field is importable that way (it creates Website entries), it just needs a column literally named url, website, site, link or address. Same idea for Notes (notes, note, comment, description, memo).
A single find-and-replace on the first line of your CSV applies this to all several thousand records at once, no per-entry work. Note there's no per-vault "default custom field name" setting but with native URL/Notes columns you won't need one. (Any unrecognized column just gets appended into the entry's Notes.)
If you genuinely need named custom fields at scale, Remote Desktop Manager connects to the same workspace DPM reads from and adds two tools the standalone import doesn't have: the CSV Wizard + Entry Templates (map columns to named custom fields), and the PowerShell module (New-RDMEntry / Set-RDMEntry) to script thousands of entries with exact field titles.
Best regards,
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