Customize seller login URL: Multi-Vendor Marketplace for Shopify

• Log in to your web hosting service. Here, we’ll take an example of GoDaddy.
  After login, go to My Products section and then click on DNS as shown in the image below.

Visit your Shopify backend > Online Store > Domains.

Under the heading Shopify-managed domains, click on Manage.

On the redirected page, click on DNS settings.

Furthermore, you need to click on Add custom record.

Then select CNAME record from the drop-down options.

Enter the subdomain name that you want. For example, if you want your subdomain to be example.mystore.com, then enter ‘example‘.

Once confirmed, your CNAME record will be created, refer to the image below:

Further, if you want to enable SSL for the seller subdomain, you need to use a cloud-flare name server. For that, click on Change nameservers under the heading Shopify’s Nameservers.

Choose custom nameserver and now follow these steps: Instructions to enable SSL for seller subdomain

Firstly, you need to create an account on Cloudflare. (It’s free!)

Cloudflare, Inc. is an American web infrastructure and website security company, providing content delivery network services, DDoS mitigation, Internet security, and distributed domain name server services. 
Create Account: https://dash.cloudflare.com/sign-up

Now, you need to add your site by clicking on +Add a Site, if the site is already added, it will be visible either in Active or Pending state:

If in a pending state, click on the site & you will be redirected to the following page. Here, you will have steps to complete the nameserver setup. For this, login to your web hosting service, remove the given nameservers (upper highlighted box in the image below) with the Couldflare’s nameserver (lower highlighted box in the image below).

After this, your site will be active. Now, click on the site as shown in the screenshot below:

Furthermore, on the redirected page, click on DNS. Refer to the image below:

After this, you need to copy the nameservers and then add this to your already hosted server like GoDaddy, Shopify, etc.

Note: Once you move to the Cloudflare and add the site to make it active, kindly check for the seller’s entry in your DNS. If there is no entry for the same, add it manually.

Make sure SSL/TLS encryption mode is set to be flexible. Set this under: SSL/TLS > Your Shop > Account > Cloudflare – Web Performance & Security:

Lastly, you need to create two Page Rules. For that, click on the Page Rules menu as shown in the image below:

On the redirected page, click on Create Page Rule as shown below:

Using these steps, create the following page rules:

• Always Use HTTPS
  With this, any http:// URL is converted to https:// through a 301 redirect.
• Cache Level: Bypass