Doing an action when focussing out of a field

[Steve King]

Hi All

I know this is going to be really really basic, but I haven't used Rev for a
couple of years and cannot get this simple thing to work

Enter characters into a field
When losing focus from that field, take the content of that filed, add
contents of field B and put into field C. A bit spreadsheet like

I have tried OnFocusOut ... Put.. but the target fld "C" is not updated! I
have also tried with MouseUp and OnReturn still target Fld C does not update

Cheers
Steve