The first thing you will have to do is going to this link and logging into your Shopify account. You will see the Setting section in the bottom left corner of the screen. In this section, choose Shipping which is between the Checkout and Taxes section.

In the Shipping details screen, scroll down, and you will see the Dropshipping and fulfillment services at the bottom of the page. Afterward, click Add dropshipping services to activate a service. In case you have already added ones, and now you want to add another, you can click on the Manage services button instead.

In the Fulfillment and dropshipping screen, when scrolling down to the bottom of the page, you will see the Custom order fulfillment section. Click on the Add custom fulfillment service which is on the left side of the screen.

In this step, a small dialog will appear, you will need to enter the information about the title of your custom fulfillment provider and the email address of them.

After entering all the necessary information, you need to click Add fulfillment service to save the details you have entered.