How do I add a prefix to a calculated value?

Im using the calculator option, I want to add a prefix to the final value, How can I add it?

This can be done by simply adding the sign in the calculation wizard: 

But do note that you have a theme applied to your form and you may notice that the wizard and other icons are missing there. 

To get the icons again, please remove the current theme by clicking on the "Themes" icon and then clicking on the button "Remove": 

Save the changes, go back to the Form Builder and add the change to the Form Calculation widget by clicking on the wizard icon: 

Once you have finished with this you may only go to the theme section and apply the theme again, the result should be the same as you've not injected CSS code to customize the layout. 

Please if you have any question, let us know.