ScriptRunner: Cannot access value of other fields in Initialiser

[Yifan Guo]

Project ID: This is a numerical project ID that the user enters. 

Task ID:

This calls an external REST service that returns a list of Tasks belong to the above project ID.

I have defined a REST endpoint where it takes in the projectID from the query parameter and it is working fine.

https://<jira-home>/rest/scriptrunner/latest/custom/getMLTasks?query=<projectID>

However, in the initialiser for TASK ID, when I am trying to retrieve the value that the user selected from the Project ID field, it keeps returning null. Please note that this project ID has not yet been created. I need this to work during a create/update issue dialogue.

Task ID (initializer code):

import groovyx.net.http.ContentType
import groovyx.net.http.HTTPBuilder
import groovyx.net.http.Method
import org.apache.log4j.Level
import org.apache.log4j.Logger
import com.onresolve.jira.groovy.user.FormField
import groovy.transform.BaseScript
import com.onresolve.jira.groovy.user.FieldBehaviours
import com.atlassian.jira.component.ComponentAccessor
import com.atlassian.jira.issue.IssueManager

@BaseScript FieldBehaviours fieldBehaviours

def projectID = getFieldById("customfield_12426").getFormValue()

getFieldByName("MavenLink Tasks").convertToSingleSelect([
    ajaxOptions: [
        url : getBaseUrl() + "/rest/scriptrunner/latest/custom/getMLTasks?query=${projectID}",
        query: true, // keep going back to the sever for each keystroke
        minQueryLength: 3,
        keyInputPeriod: 500,
        formatResponse: "general",
    ]
])

I have also tried putting the above code as a server-side script for the Task ID but it seems that convertToSingleSelect does not work anywhere else except for the initialiser section.