do I understand correctly that there is another attribute in that entity which is called “reference” (string attribute) and you want to add to that string a date?
If yes, you first do the check if the boolean is true (so what you are already doing). Then you do the change object action and select the object for which you’d like to change the reference attribute. In the “Change Object” activity you then select “New” and select the “reference” attribute.
Inside the editing window you then put this for example:
$<objectname>/reference + formatDateTime([‘CurrentDateTime’], ‘dd.MM.yyyy’)
Where <objectname> is the Object you selected to change itself and formatDateTime puts your date into a string format.
Hope that makes it clear to you!