I have found this documentation about the BSTR data type:

   https://learn.microsoft.com/en-us/previous-versions/windows/desktop/automat/bstr

Essentially, the BSTR data type is a pointer to a Unicode character string, same as a PB String data type argument. One thing that might concern me is that this documentation mentions that the BSTR data string may contain imbedded null characters. If the BSTR's your DLL produces do this, you will not be able to obtain all of the information without a memory copy into a PB Blob because the first null that is encountered will mark the end of the string.

If the data string contains a "normal" character string that does NOT contain imbedded null characters, I think you should be able to use the standard PB String data type in the external function declaration.

If this is a Windows API function, what is it?

I looked it up and BSTR is a Visual Basic String. If this is a VB DLL, it should be deployed as a COM and accessed from PB using OLEObject.

well it is a dll used to communicate with a fiscal memory device.

From the documentation:

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9. DLL specification
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  * The functions have a PASCAL calling convertion (same as win APIs).
  * The functions provided by this DLL are *NOT* reentrant.
  * Many of these functions are blocking.
  ...