I would use css to add the percentage sign to the text field.
A working fiddle - https://jsfiddle.net/pbjsyrax/
If you do not want to use the outcome for any further calculations, you could consider using a string field in stead of an integer.
The eventhandler is now calculating the integer field (eg value_a*value_b) value_a and value_b both being 5. For the string field you could use toString(value_a*value_b)+'%' (see https://docs.mendix.com/refguide/to-string for more information).
That way it should show up as 25%
Hope this helps