Expand the columns which the Set Regarding lookup searches on
To have the Set Regarding lookup search on custom columns in Dynamics 365 App for Outlook, that column must be added to the table’s quick find view.
Sign in to Power Apps.
On the left nav, select Solution.
Select the solution that contains the table and then select Edit on the command bar. For more information on solutions, see Solutions overview.
On the command bar, select the All and then select Table.
Select a table such as Account to edit the table.
Select Views and make sure that Quick Find Active Accounts view is active. If it's not, then open the view and select the necessary find columns.