Add customfield value as a NEW Option Using REST API - JSON

I want to insert a custom field value into the JSON using addOptions. Is this possible?

Here is what I have:

string fullname = customfield_10542; //grabs the value of the field

string addOption =    "{" +
                        "\"options\": [" +
                            "{" +
                                "\"disabled\": false," +
                                "\"value\": \"fullname\"" +  
                            "}" +
                        "]" +

What happens when I run this is the text “fullname” gets added as an option instead of the customfield value. I know this sounds somewhat elementary and it is easy to see what is happening. I was hoping for a pointer into how to inject the customfield value into that JSON script.