You have to add a "Related Objects List" and configure it to show the CIs (The option is called "Flex Settings" not the most clear name if you ask me, but...):
It will show like this on your form:
Of course this was a very quick and dirty implementation for the sake of the example, play with the list options to find something that works for you (ie. uncheck the "Delete" option as it would allow to delete the CI straight from the list... dangerous!)
Let me know if you find issues.
thank you so much for your answer. The reason why i need a field displaying linked CI is to use this field when i export Incidents. I dont know if a related objects will allow me to do this