You should use the value for the desired choice.

For example, your lookup field is connected to a text field on the source form, named Employee Name. You have a record on the source form where the value John Doe is submitted for the Employee Name. If you want to submit the lookup field connected to this field (Employee Name), you should submit this:

{ 'lookup_field_slug': 'John Doe' }

This will be the title field in the choice_item object on the lookup field's data.

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