The Product Seller Required certain details so connect the Magento Multivendor Store to their Shopify store and fetch the products and their detail, for that the API Key and Rest of the required details can be retrieved from  the below mention

Retrieving the API Key, Password, and Shared Key

• Go to your Shopify Seller Account.
• On the home page, you can see Apps on the left side menu, as shown in the red box in the image below:

• Click on the Apps and you’ll be navigated to the page as follow
• Click on Manage private apps, as shown in the image above

• On clicking it, you’ll be navigated to the page as shown below

• Click on the private app name which will be listed as shown in the image.
• On clicking it, you will be navigated to the page as shown

From this page, Seller can collect your API Key, Password, and Shared key to use them while doing the Configuration.