Adding contacts from the account to the email activity

Dear,

On the email page, I added a button that displays a modal window via a process and shows the recipient's account contacts.

This is to allow users to select contacts from the account and add them as recipients to email activities.

But when you click the "add contacts" button, the contacts are not added to the "to" field of the email.
Here is the process:

I tried adding an action to modify the activity and add the selected contacts, but I don't see how to retrieve the list of contacts selected by the user.

Has anyone ever successfully performed this operation ?

Thank you !
Nicolas