Good afternoon,
I am creating a Sierra type adventure game and I want a character's response to be displayed within a text box.
Thank you so much
In General Settings -> Dialog -> Speech style, pick "Sierra-style with background".
This puts the default text box behind the speech text next to the portrait.
If you want it to look different or customize it further, please tell us exactly how you want it to look, ideally with a screenshot from an existing game or something like that.