Find custom field ID

There are a few ways to find the custom field ID:

Old Issue View

Open the Old Issue View page (available from the "..." menu in the top right corner of the New Issue View page). 

customfield_10159 visible on the screenshot below is the ID you are looking for:

Jira settings

Steps to get Jira custom field ID if you can access Jira administration section:

• Navigate to Jira Settings >> Issues >> Custom Fields.
• Click the cog (three dots menu button) and hover over Configure or Edit details option.
• Observe the URL at the bottom left of the browser window. Example:
  
   The id for this custom field is customfield_10006.

REST API

The API endpoint that allows getting the list of the custom fields along with their IDs: 

curl https://Your-Jira-URL/rest/api/2/field

Reference: https://developer.atlassian.com/cloud/jira/platform/rest/v3/api-group-issue-fields/#api-rest-api-3-field-get