After browsing this forum not without difficulty, being French, I found some ideas there and now I can launch AP2 from another application (DXO Photolab).
Here's how I did:
Go to C:/programs\Affinity and create a new "Photo2" directory, or whatever name you want
nglais
Then go to: C:\Program Files\WindowsApps\SerifEuropeLtd.AffinityPhoto2_2.0.0.1640_x64__3cqzy0nppv2rt\
You will need to have the necessary rights, and if necessary create them.
Copy everything in this directory
Then paste in the "photo2" directory that you created.
The Photo.exe file is located in the directory: C:\Program Files\Affinity\Photo2\App and is now accessible from your other favorite photo editing software.