Once you buy the SSL, To activate the SSL you have to provide CSR key
To generate CSR, use the following command
openssl req -nodes -newkey rsa:2048 -keyout mathan_xyz.key -out mathan_xyz.csr
here mathan_xyz is the domain name. Now Submit your CSR key to activate your SSL.You will get a .zip file which has 3 files in it.
Now, we have to create a .pfx file which you have to upload it on azure.
Use the above link to generate .pfx file.
Then, go to your azure web app → TLS/SSL settings, click on Add binding and upload the file.
Its done. Now your site is secured!!