How to set default value and dynamic value on same field

Hello All

we have 1 field called description  on portal and i set some default value there by using default field on variable form.

There is another field called user name (string type) so for example

ITIL user type  abc , def in User Name field so when ITIL user submit the form so on the record in ITIL view I want description field to populate default value + abc , def (user field values).

so the description field  looks like:

      abc , def 

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