Lookup Data from Another App

I have a table in a new mentoring app that grabs information from a separate app on employee separations. Basically, I want an Excel XLOOKUP function so there is notice when one of our mentees and/or mentors is separating from the company. I have the key set to the Employee ID, which is also the lookup field for the app to app connection. It does what it's supposed to in that regard. However, when I go to add a new mentor or mentee, I can only select an Employee ID from the list of separating employees, instead of adding a new value for the new record. Is there a way to get around this restriction and still get the info I need from the other app? 

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Hannah Engelson
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