You can do this with yad
, which you can install from the Software Centre. You can run:
yad --title "Customised title here" --entry --text " Are you <b>really</b> sure you want to add an entry?" --width=300 --center --button="gtk-cancel:252" --button="gtk-ok:0"
To make this usable we need to get any data that was inputted and check which button was pressed. This does it:
data=$(yad --title "Customised title here" --entry --text " Are you <b>really</b> sure you want to add an entry?" --width=300 --center --button="gtk-cancel:252" --button="gtk-ok:0");rc=$?; [[ $rc = 252 ]] && data="";echo $data
For help on yad type yad --help-all
or man yad
but I found practical examples are easier to adapt. There are some here.