How to search in a specific field with REST API

Question

I created a fake REST API. My user object like the following:

users:[
 {
  id:1,
  firstName: "Terry",
  lastName: "Max",
  ...
 }
]

I want to query only the users which have the firstName included Tr characters.

When I search with this, it searches inside of every properties like firstName, lastName etc.

http://localhost:3000/users?q=Tr

How can I search this text only in firstName property?

By the way where can I find a full documentation of REST API? When I search it on google, I found someone else's APIs.