Hi @jk_br and thank you for tagging, @CateDuarte.
I would also suggest creating two properties, one for use in the form (1), another one (2) to be populated in a workflow by appending the value from the form property.
Forms do not have an option for appending values, although much requested: Append new values to multiple checkbox property instead of overwriting previous values
To make sure that only property 1 is used in forms, I’d recommend updating the property setting for property 2. In the property settings, you can choose to not make it available in forms – preventing users from accidentally using it in forms.
Cheers!