Environment/ Global variable in customization.xml file

Hi All,

Scenario: I am using MS CRM 2013 on premise. 
I had some custom web application deployed on different website with different port. I am using that custom web pages on CRM form using iframe. Here i my giving URL for custom web pages.

Question:: URL for Iframe is going hard coded with absolute path. I dont want to hardcode URL here. Is there any other way to do that? Can we use environemental/ global variables in customization.xml file.

If anyone need more details please let me know.

Thanks in advance.