To make this work you need to point the CNAME record with your subdomain (proposals) to myaccount.niftyquoter.com (replace "myaccount" with your actual subdomain!) in your domain's DNS settings. And then, when the CNAME record has been successfully added, you need to fill in proposals.mycompany.com to the custom domain field in your account settings in NiftyQuoter.
You can add CNAME record to your domain DNS settings either by yourself or you can contact your registrar/webmaster and they should be able to do this for you.
How to do this by yourself:
Google has a nice guide how to add CNAME records for different most popular domain registrars: https://support.google.com/blogger/answer/58317?hl=en
If you'll follow Google's guide you want to use these values given our example situation outlined above:
|proposals||myaccount.niftyquoter.com (replace "myaccount" with your actual subdomain!)|