Thank you for reaching out and posting this question!
The merchant ID is something that our live support can assist the store owner with, so you'll need to have your client reach out to us through the Shopify Help Center. From there, we'd be happy to assist your client through live chat, email, or callback.
That being said, as you've mentioned the "merchant client secret no", then I believe you're also trying to integrate a custom app where you can generate API credentials (eg. secret key). If so, then you can follow our guide to help your client generate the credentials needed to integrate the "cred pay app".
I noticed that you're a new member of our Shopify Community! Since you've mentioned that you're helping a client, then have you signed up to be a Shopify Partner?
Log into https://merchants.google.com/. Select the right account, then from the top-right is the ID near the account name.
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