I have this table, which has a field "Codigo". Basically is a table to set stages of sales opportunities and "codigo" is an ID defined for each stage

In the first stage, the user insert the opportunity and i would like that the "codigo" was set automaticly as "CCV0" whic is the code for the first stage. But I would like to hide this field from the user( I just keep it visible to test), so I want to make it automatic

Actually I've tried in many ways, and this was the last one:

So in this form I would like to set "CCV0" as default for this moment, but nothing works and this is the error: