You can achieve through css styling, add the classname currencyTextbox to your text box and update your custom.scss file and compiler(https://www.dropbox.com/sh/46hip6pq8s9ipqp/AAAHBwm-8qcv9bQKSR7BrirCa?dl=0) your custom.scss file using calypso compile with the below styling
content: "$" !important;
If it is not working the problem is with the css compilation, for testing this style you can also add the style in your main.css file
You can also use Bootstrap Input Addons widget from app store. If this fits your requirement than you will not need any custom styles.
Hope this helps!
As an alternative, you could also implement the input field as input group by simply reusing some of the Bootstrap classes. Examples can be found at https://getbootstrap.com/docs/3.3/components/#input-groups-basic