This page describes how to create a custom field in Salesforce to associate records with an external ID for use with Jitterbit.
Creating a Salesforce Custom Field / External ID for Jitterbit
These steps show how create a custom field in Salesforce on the Account object to create an external ID called Jitterbit_Account_ID. The steps use the Salesforce Lightning Experience; similar steps can be applied to the Salesforce Classic view.
Log in to the Salesforce instance with your credentials.
Navigate to Setup > Platform Tools > Objects and Fields > Object Manager.
From the Object Manager tab, select the standard Account object.
From the Account object view, go to the Fields & Relationships section, and click New in the top right.
In Step 1. Choose the field type, select Text as the data type and click Next.
In Step 2. Enter the details, configure your new custom field as follows. Then click Next.
External ID: Checked (It's essential that this be set for the field to function as an external ID.)
In Step 3. Establish field-level security, accept the default settings by clicking Next.
In Step 4. Add to page layouts, accept the defaults by clicking Save.
You are returned to the Fields & Relationships screen for the Account object. Verify that you see the new custom field with the same parameters as shown below: