The Customer Request Type Custom Field is easy to configure. All you have to do is make a custom field and add this custom field to the screen of your JIRA Service Desk.
For a general description on how to make custom fields you can read the Atlassian document https://confluence.atlassian.com/display/JIRA/Adding+a+Custom+Field
Step 1:
Go to 
> Issues. Select Fields > Custom Fields to open the Custom Fields page.
Keyboard shortcut: g+g start typing custom fields.
Step 2:
Click the Add Custom field button. The following dialog is displayed:
Choose Advanced. The following dialog is displayed:
Choose Customer Request Dummy Field. Click Next. The following dialog is displayed:
Type a name which suits your project. Click the Create button when you are ready. After this you will get the page "Associate field Choose the customer request Type to screens" in which you can choose on which screens the custom field should appear. Please choose the screen which is being used for the creation of your Service Desk issues.