Before you can add a custom field for Jira Service Management (JSM) projects, you first need to setup the custom field. See Create a Custom Field .
Step 1: Switch to the Jira Service Management Tab and Chose the Request Type
Find the request type you want to add your custom field(s) to and click on EDIT FIELDS.
Step 2: Configure Request View
Drag & drop your custom fields into the blue zones of the request form on the left.
Step 3: Configure Request Form
Switch to the FORM tab and drag & drop your custom fields into the blue zones of the request form on the left.
Step 4: Add the Fields to The Issue View in Jira Configuration
In your projects settings find the tab REQUEST TYPES. Chose the request type you added the custom fields to and drag & drop your custom fields to your desired position.