Fill-in the card as usual... then click the Update share and select policy button. The bot will reply with another card, allowing you to select or modify the default policy:
From here you can modify the policy by checking or unchecking the various options... then click Update policy. If you want to modify the expiration, click that button for a special card allowing you to set the expiration (including one-time access).
You can also Select another policy from your organization's official list:
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