Hi Dominique - Welcome to the Atlassian Community!
You would start with an automation rule with a trigger of Issue Transitioned to Done.
There needs to be some type of linkage between the issues. Once you identify that with a Branch, you would do a new action of Edit Issue and select the Due Date.
Then in that field you could use the formulate of:
First of all, I'm going to introduce myself. My name is Gorka, I work as product manager in Appfire Although this is my first public article in the community, I've been around since 2013. I starte...
Connect with like-minded Atlassian users at free events near you!Find an event
Connect with like-minded Atlassian users at free events near you!
Unfortunately there are no Community Events near you at the moment.Host an event
You're one step closer to meeting fellow Atlassian users at your local event. Learn more about Community Events