Custom module - update the email address field when a contact is selected

I've read a few articles around updating fields automatically but I really couldn't wrap my head around the code. Could you please help me?

Module name: Contracts

Field to select: Technical Contact

Field to update: Email

What i'd like to do is update the email field automatically when I've changed the Technical Contact field. I read that I need to populate the email field in order to make to make use of the "Send Email" button. (I know I can use the workaround for a mail merge but I didn't want to do that)

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