Some plugins for forms have the IP address stored by default. If you are using some kind of plugin for that, you just need to check the form's code if it receives the IP address, there you should be able to set the new line for you to receive, similar to "[_remote_ip]" or "[show_ip]".
If there is no function like this, then the recommendation would be to get the new plugin for your form or add the additional PHP function to the code you are using currently.