In FileMaker everything happens "in context".
- Context
 - Context is Everything
 - David Attenbruh's World of FileMaker™ - Episode 1 - Context
 - Best way to check FileMaker “Context”?
 - Layout Context
 
In your case, everything happened in the context of the current record on the current layout. The first instruction did a lot less that you expected. It went to the first portal row but it remained in the current context. When you told it to make a new record that happened in the current context, not in the related table.
Your script needs to do this:
- Copy the current record ID into a variable
 - Go to layout based on the related table
 - Create a new record
 - Use the value of the variable to set the foreign key to the establish the relationship
 - Go back to the original layout
 - refresh the portal
 - go to field [ dépôt provisoire ]