As of the 7.x-3.x version, fields may be attached to Flags. This allows users to enter additional data when flagging a node, user, comment, or other entity.
Attaching fields to Flags
Log in as a user with the "administrator" role. Navigate to Administration > Structure > Flags. Here, you are presented with a list of Flags that have already been defined, one per row.
Click the manage fields link in the row for the Flag to attach fields. From this point forward, adding and configuring Flag fields is similar to adding and configuring fields on nodes.
Using Flag Fields
Navigate to Administration > Structure > Flags page. Click the Edit link for the Flag. Scroll down to the Display Options box and select the Confirmation Form radio button.