Add a relationship on report issue

Hi,
I would ask how can i make a customization.
I have to add a custom field in the report issue page where the user can 
insert a bug reference id.
When the user submit report I want to create a relationship (custom) like 
this: the created issue is related to the reference issue.
I hope I was clear.

PS. I know how to create a custom field and how to create a custom 
relationship.

Best regards 

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