Hi Gabriel
If I understand correctly, you are looking for an option like shown in below screenshot :
In order to achieve this, you will need to create new web application using default unsubscription form and then customize the web application to include either a text area or check box to grab the reasons and assign a field where you want the data to be saved.
In above example assigned a comment filed to this textarea check below :
In addition to this, you may want to go through below article to understand how you can manage subscriptions and un-subscription :
https://docs.campaign.adobe.com/doc/AC/en/DLV_Subscriptions_and_referrals_Managing_subscriptions.htm...
In case you want to create you own custom web form , you may follow the article : https://docs.campaign.adobe.com/doc/AC/en/WEB_Web_forms_About_web_forms.html . This would guide you to create a new web-form from scratch.
Let me know if this approach helps.
Regards
Anshul Sharma.