The Grouping Input Digits Widget (https://appstore.mendix.com/link/app/116191/) has a possibillity for placing text before or after a inputfield.
Also the widget Bootstrap Input Addons (https://appstore.mendix.com/link/app/2790/) is an option.
If you don't need an input widget, you could also use Text instead of a TextBox and add it in like this:
ohw nice my first question and in no time two great answers ! thanks