To get your Amazon ID, you have two options:
Option 1) Click on the link: https://sellercentral.amazon.com/sw/AccountInfo/MerchantToken/step/MerchantToken?ref_=macs_aimertok_cont_acinfohm&
You will be redirected to log into your Amazon Seller Central Account and as soon as you log in, you will be able to check your Amazon Seller ID (Merchant Token), as below:
Just copy and paste it in the proper field.
Option 2) Amazon SellerCentral navigation:
You need to go to Amazon's Seller Central: https://sellercentral.amazon.com
Click on Settings (top right corner), and on Account Info.
You will be directed to your Account Info screen.
Click on "Your Merchant Token".
You will be taken to your Seller ID (Merchant ID), as below:
You will see your Seller ID (Merchant Token ID).
Just copy and paste it.