Can't update field with behaviour on create screen

[Elena Oleksenko]

Hello!
I need to update a default value for Description field when the option is selected in another field (Select List (cascading)), when creating issue. 

I created the behaviour and added the field that should be checked when changed, to this behaviour. 

But when I'm trying to create an issue and select the needed option in the Select List field, description field is not updated with the default value. 

Here is my script: 

import com.atlassian.jira.component.ComponentAccessor 
import com.atlassian.jira.issue.CustomFieldManager 
import com.atlassian.jira.issue.fields.CustomField
import com.atlassian.jira.issue.customfields.manager.OptionsManager
import com.atlassian.jira.issue.ModifiedValue

def targetField = getFieldByName("Description")
def defaultValue = """[TASKS|]""".replaceAll(/ /, '')
def cField = getFieldByName("Deployment flow")
def selectedOption = cField.getValue().toString()
if (selectedOption == "PHP"){
targetField.setFormValue(defaultValue)
}

 Please help to understand what I'm doing wrong and how to update description when the option is set in my custom field. 

[Roland Holban (Adaptavist) · Accepted solution]

Try to put your behaviour script on the Select List (cascading) field rather than in the initialiser