Jay,
You need to add code to your form in the \widgets\Print\templates\Print.html file
It depends on where you want the user to enter the information. Visible from the top of accessible through the advanced button. For example I wanted mine to show up in the advanced section so I just added this between the Author and copyright table rows.
<tr>
<td>
Additional Text:
</td>
<td>
<input type="text" data-dojo-attach-point="BOXTEXTNode" data-dojo-type="dijit/form/ValidationTextBox" data-dojo-props="name:'BOXTEXT',trim:true,required:false,style:'width:100%;'" data-dojo-attach-point="BOXTEXTTB"/>
</td>
</tr>
In line 6 above replace the 3 instances of BOXTEXT with the value you made in the Print.js file as mentioned previously.
-Stan