Create a Merchant ID on Apple Developer

You must have an account with Apple Developer to create a Merchant ID.

Login to Apple Developer.
Go to Certificates, Identifiers & Profiles.
Click the Identifiers tab.
The identifier management page opens.
Click Merchant IDs in the App IDs list.
Then click the + icon to the right of Identifiers.
The merchant identifier registration page opens.
Check the Merchant IDs radio button and click Continue.

It is recommended create two distinct Merchant IDs, one for your test environment and one for production.
Fill in the Description field if you wish.
Example: merchant id demo if it is your test account.
Enter your Merchant ID in the Identifier field.
You must choose your own ID according to Apple’s recommendations.

Example: merchant.com.mycompany.demo if it is your test account.
Click Register to complete the process.
Your Merchant ID is created.