This scenario is part of Actions Example 12: Generic Search Dialog.
To get a dialog that can act as a generic toolbar and function with ANY calling window:
Rename and adapt your FindExistingReservations Dialog (Example 11) into this newly defined SearchDialog.
A lookup button will appear that calls a lookup window for Tours.
Make sure to select the SearchDialog in the Object Browser tree view (top-left) while defining those properties. Note that the first property is no longer of the Reservations type (like in example 11), but the more abstract Info Window type. The other two properties have the BaseColumn as the type which can refer to the query columns in another table. Note that instead of BaseColumn, you can also use the OUTPUT_COLUMN type with the same result.
In the second and third line, you need to be specific about the Value property to avoid type mismatch.