In my case, I had a single text field that contained a default text calculation that had HTML. When Formidable started using wp_kses to prevent unwanted HTML in fields as WordPress does, my HTML got stripped. After some investigation by Victor M Font Jr (thanks Victor), it was discovered there were not any hooks or anything available to 'undo' this.

I was trying to avoid changing my field type to HTML, as I would have had to create a whole new field and lose data for hundreds of entries.

So I changed my text field to rich text (which allows some HTML). Then I used the frm_validate_field entry to construct a string of field values and text.

This worked for me because I really did not need the field to be populated as 'default'. It was a field the users were not seeing and just needed to be populated to get passed along after the create entry/post...

If you do need to use default - you will have to do the HTML field - Javascript method as outlined in the Formidable documentation.