Hi,
Copy the following code in your ACADDOC.LSP file. If you don't have one, just create it. If you have one and it contains a S::STARTUP function, paste it inside. The file should be in the SUPPORT folder of your AutoCAD install directory.
I wasn't able to find this registry key one neither of my AutoCAD installs (2006 French and English, 2008 French and English, 2009 English, 2010 English) and although I would suggest something more general, like
vl-registry-write
(strcat
"HKEY_CURRENT_USER\\" (vlax-product-key) "\\Profiles\\" (getvar "cprofile") "\\Dialogs\\XrefFileDialog"
"InitialDirectory"
(getvar "dwgprefix")
)
)
maybe it's a better idea to try this hard coded version (copied from your posted image) first:
(vl-registry-write
(strcat
"HKEY_CURRENT_USER\\Software\\Autodesk\\AutoCAD\\R17.0\\ACAD-5001:409\\Profiles\\<<Unnamed Profile>>\\Dialogs\\XrefFileDialog"
"InitialDirectory"
(getvar "dwgprefix")
)
)
and if it works, you can try the first one too.
HTH