Same thing that happen in QUOTE screen in reality...

Or would it be possible to have a field "customer" displaying the customer from the requester?

i would like to display customer on a ticket, retrieved automatically from the concerned project, but it does not work, only a drop down list is available with all customers. How to proceed please ?

I guess you added idClient to the screen, so you have the list.
But there is no possible automatisme to select customer from the project (or from the requestor) without some coding.

Possibly you can manage with default value...
- set field as readonly
- entrer default value as

###EVAL###(new Project($this->idProject))->idClient