Pretty certain this can be achieved using action framework in row actions. ie. set the field to be equal to {{LONGTEXTAREAAPINAME}}, {{DesiredTextFieldFromLookupRecord}}.
I’d also create add a button to “Restart/Clear” the text box in case of miss click.
Might be able to use the same action framework on a Mass Action. It would be a better user experience to able to select the desired lookup records until ready and then click “Submit” to populate the text area field.
