Dynamic Default value for Fixed Choice List

Summary:

In Fusion CX sales, I have created a Fixed Choice List field in that I want to default the value based on the roles of the user.

For Example:-

Field Name Account Type

For Roles as 'Sales Manager' :- Account Type is 'Customer'
For Roles as ' Sales Manager Customer' :- Account Type is ' Star'.

So in that I am trying to default a value based on the Expression but I am unable to select the Expression field.

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