Add a Trigger → Issue Event → Issue Created

Add the following expression:

%{trigger.issue.priority} = "High"

After importing the JSON file, make sure to check the configuration of the rule. Non-existing configuration elements (issue types, fields, values etc.) will be highlighted.

{
    "name": "Notify for high priority issues",
    "description": "",
    "creator": "admin",
    "status": true,
    "triggerData": "1",
    "triggerType": "ISSUE_EVENT",
    "configuration": {
        "refs": [
            "issue",
            "system",
            "trigger.issue",
            "trigger.parent"
        ],
        "triggerType": "1"
    },
    "children": [
        {
            "sequence": 0,
            "type": "BOOLEAN_CONDITION",
            "ruleEntityType": "CONDITION",
            "configuration": {
                "refs": [
                    "issue",
                    "project",
                    "system",
                    "trigger",
                    "trigger.issue",
                    "trigger.parent"
                ],
                "expression": "%{trigger.issue.priority} = \"High\"",
                "expressionParsingMode": "logical",
                "actingUser": "field_00020"
            },
            "children": null,
            "hasChildren": false
        },
        {
            "sequence": 1,
            "type": "SEND_MAIL",
            "ruleEntityType": "ACTION",
            "configuration": {
                "refs": [
                    "issue",
                    "project",
                    "system",
                    "trigger",
                    "trigger.issue",
                    "trigger.parent"
                ],
                "fromOption": "fromUser",
                "fromUser": "admin",
                "toUsersInField": [
                    "00003"
                ],
                "toUser": [
                    "admin"
                ],
                "subject": "%{trigger.issue.key} = \"is of high priority\"",
                "subjectParsingMode": "textBasic",
                "contentType": "text/html",
                "mailPriority": "3",
                "sendingMode": "common",
                "body": "Please take care of this issue first. \r\nCheers, \r\nAdmin",
                "bodyParsingMode": "textBasic"
            },
            "children": null,
            "hasChildren": false
        }
    ],
    "hasChildren": true
}