Hello Frank,
Thank you for your reply. I just tried with the tutorial, finally we add these tags
<desc>[url=http://my.domain.com]Link text[/url]</desc>
in the file gpx. It works perfectly.
I noticed that the behavior of the popup window is different with the validation of the symbol or not. If I validate the symbol, the flyover window remains displayed, otherwise it is not the case.
Best regards
Bernard