concat two fields and store the value in a third field

Scriptcase 9 General
#1

I have 3 fields; all are varchar - although one of them is only allowed to contain numbers. For example:
FIELD1 value = MIB
FIELD2 value = 120

I want FIELD3 value to be “MIB120”.

Using an ajax onchange event {field3} = {field1} + {field2} does not work.

How can I concat field1 & field2 and store the result in field3?

#2

{field3}= {field1} . {field2};

#3

Sometimes the simplest things evade me. Thank you.