Unless I misunderstood your question, you get the shopID in the install URL when the merchant installs your app.
This is the default form merchant get when installing the app and not logged in. You cannot have custom intermediary login pages as per OAuth requirements. On a side note, why would you need that?
@Visely-Team How can I get the shopID during installation. Currently when I'm developing the app, I'm installing app by directly providing my store url in the install url in my server which does the oAuth process and all.
This is an accepted solution.
Once you publish the app on the Shopify app store all users will get this screen and Shopify will redirect to the installation URL you provide in app setup passing the storeID. You don't have to do anything extra.
@Visely-Team So you mean if my install file is like https://mydomain.com/install.php then during app installation shopify will pass the shop parameter like https://mydomain.com/install.php?shop=mydomain.myshopify.com
shopOrigin is appended to your install URL when a merchant clicks install from the app listing. You get it from the query string and use it to build the callback URL, etc.