It’s way at the bottom of the inbox settings.
Settings > Inboxes > Inbox
From here, choose the inbox in question from the Current View dropdown on the top of the page.
Then, in the Channels section of the page, a list of the Connected Channels will load.
Find the shared email inbox you want to update and click Edit.
Scroll to the bottom of that page to find the “Choose original sender of email forwards” setting.