I finally found where I can find the promoted URL of my Ad.
In fact when you generate your Ad through the Web Ad Manager it will create a hidden post on the Facebook page that is used for the Ad (see this blog post). And Ad creative use an object_story_id and object_story_spec to link to it. So I can find my link in the field 'object_story_spec' of the creative.
->/v2.3/6024899447563/adcreatives?fields=object_story_spec
result:
{
"data": [
{
"object_story_spec": {
"page_id": "1574237802836192",
"link_data": {
"link": "http://www.mylink.com/destination",
"message": "Click!",
"name": "My test",
"description": " ",
"image_hash": "9b25eba026f1ad716010963db25fabb8",
"multi_share_optimized": false
}
},
"id": "6024899447563"
}
],
"paging": {
"cursors": {
"before": "NjAyNDg5ODQ0ODE2Mg==",
"after": "NjAyNDg5OTU0ODE2Mg=="
}
}
}
So the link is in object_story_spec.link_data.link of the creative of the Ad.