How to impersonate the user in Jira add-on to use Jira REST API on behalf of user - jwt

Question

I'm looking for help of somebody who knows something about Jira add-ons.
I'm trying to implement Jira add-on with an ability to create issues on behalf of a user.
So, I completed all steps (as I think) leading me to my goal, but I faced a problem with impersonating. When I try to create an issue using Jira REST API, I receive the following error:
{
"error": "Add-on 'com.example.myapp' disallowed to impersonate the user because 'no valid active user exists'"
}
What was done:
Folowing the getting started tutorial I created the add-on descriptor:
{
"name": "Hello World",
"description": "Atlassian Connect app",
"key": "com.example.myapp",
"baseUrl": "https://url.ngrok.io",
"vendor": {
"name": "Example, Inc.",
"url": "http://example.com"
},
"authentication": {
"type": "jwt"
},
"scopes": [
"act_as_user",
"read",
"write",
],
"lifecycle": {
"installed": "/api/created",
},
"apiVersion": 1
}
I also implemented webhook to handle 'installed' callback from add-on like described here. So, when a user installs the add-on the webhook will receive the following object:
{
'key': 'com.example.myapp',
'clientKey': '<client key>',
'oauthClientId': '<OAuth client ID>',
'publicKey': '<public key>',
'sharedSecret': '<shared secret>',
'serverVersion': '100095',
'pluginsVersion': '1.250.0',
'baseUrl': 'https://<user's domain>.atlassian.net',
'productType': 'jira',
'description': 'Description',
'eventType': 'installed'
}
Using this data I generated JWT signed with the shared secret like described in the tutorial. So, my JWT payload includes the following claims:
{
'iss': 'urn:atlassian:connect:clientid:<OAuth client ID>',
'sub': 'urn:atlassian:connect:useraccountid:<client key>',
'tnt': 'https://<user's domain>.atlassian.net',
'iss': '<created at>',
'exp': '<created at + 1 minute>'
}
The next step I performed is access token generation using auth.atlassian.io/oauth2/token endpoint and the following parameters:
grant_type=urn:ietf:params:oauth:grant-type:jwt-bearer&assertion=<my JWT>&scope=ACT_AS_USER READ WRITE
The result of all these actions is an access token witch I can use to perform REST API requests. But the error described above returned every time I try to create Jira issue performing POST [user's domain].atlassian.net/rest/api/2/issue/ with Authorization: Bearer [access_token] header.
What I've noticed is that account ID in installed callback object (clientKey) has 'xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx' (uuid) format. But when I open my account in Jira (account in witch the add-on is installed) I see that the account ID in URL has 'YYYYYY:xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx' format. I prepended the subject in my JWT payload with YYYYYY part:
'sub': 'urn:atlassian:connect:useraccountid:'YYYYYY:xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx'
and generated a new access token using this JWT. I've used new access token to create an issue and it was successfully created, the reporter of the issue is a user installed my add-on, just like I wished.
Finally, the question is: where can I find that YYYYYY part for my add-on? How to get it automatically? What I've missed following the tutorials? Any help will be appreciated.
Thank you!