How to use define document code

I need to change the way some of the attributes picks up input values, in the environment of one of the Database. When looking under the properties of one of the attribute the default value option is set to 'None' and under the 'Update value'  it mentions 'This field can not be triggered because it is a code field'. Instead of setting this value under code field, I want to set it as an input directly from any user. I reckon it will involve using 'define document code' but cant figure out which options to chose. If it requires different approach then kindly mention.