Update the Value of Custom Fields through the Script Console in Jira Cloud

final customfieldID = 'customfield_1000' //get this from /fields api
final newFieldValue = 'New Value'
final issueKey = 'MYPROJECT-1'

put("/rest/api/2/issue/${issueKey}")
    .header('Content-Type', 'application/json')
    .body([
        fields: [
            (customfieldID): newFieldValue
        ]
    ]).asString()