Jira REST API
You can use Jira REST API to work with the checklists ⚡️
Contents 👇🏻
☝🏼IMPORTANT NOTE: In the Forge app, updating the checklist via issue properties is no longer supported. Use the Checklists custom field to update checklists instead.
📖 Overview
You can work with checklists through two types of storage:
Checklists Custom Field – can be used for both reading and writing checklist data.→ "Checklists" Custom Field set up
com.railsware.SmartChecklist.checklistIssue Property - available immediately after app setup. It reflects checklist data, but since it’s not the main source of truth, any updates made directly to this property will not affect the actual checklist.
🔒 Authentication
Get your API token from https://id.atlassian.com/manage/api-tokens. 
Check out instructions here →  Manage API tokens for your Atlassian account | Atlassian Support 
References
🔗 Jira API reference
Then use the following approach:
Operation  | Method  | Endpoint  | Data in the request body  | 
|---|---|---|---|
Using  | |||
Get the checklist from existing Jira issue  | GET  | 
  | 
  | 
Using Checklists custom field | |||
Get the checklist from existing Jira issue  | GET  | 
  | 
  | 
Set the checklist for existing Jira issue  | PUT  | 
  | 
  | 
Create Jira issue with the checklist applied  | POST  | 
  | 
  | 
Examples
Reading com.railsware.SmartChecklist.checklist  issue property:
Get the checklist GET /rest/api/2/issue/{issueIdOrKey}/properties/{propertyKey}
Retrieves an existing list of checklists for the given issue key
Using cURL:
curl --request GET \
--url 'https://your-domain.atlassian.net/rest/api/2/issue/{issueIdOrKey}/properties/com.railsware.SmartChecklist.checklist' \
--user 'email@example.com:<api_token>' \
--header 'Accept: application/json' \
Using Checklists custom field
🔗 Jira API reference
☝🏼NOTE:  customfield_10001 custom field id might be different on your instance. Make sure you use the correct one.
Set the checklist PUT /rest/api/2/issue/{issueIdOrKey}
Set a checklist for the given issue key
Using cURL:
curl --request PUT \
--url 'https://your-domain.atlassian.net/rest/api/2/issue/{issueIdOrKey}' \
--user 'email@example.com:<api_token>' \
--header 'Accept: application/json' \
--header 'Content-Type: application/json' \
--data '{
  "fields": {
    "customfield_10001": "- ToDo List\n+ Checked\nx Skipped\n~ In Progress\n"
  }
}'
Get the checklist  GET /rest/api/2/issue/
Retrieves an existing list of checklists for the given issue key
Using cURL:
curl --request GET \
--url 'https://your-domain.atlassian.net/rest/api/2/issue/{issueIdOrKey}' \
--user 'email@example.com:<api_token>' \
--header 'Accept: application/json' \
--header 'Content-Type: application/json' Check an example implementation with ScriptRunner for Jira (Cloud)
Hope everything works for you! 🚀