Short answer: you don't.

Instead: create additional function.

Let's suppose, you have User data class.

data class User(val id: Int, val name: String, val email: String)

The easiest way to "mock" it would be to use default values. But does default values always makes sense? If it's json model, it may not be the best solution.

Create additional function createUser that will accept the same values as your data class. Now you can use "defaults" by calling that function.

fun createUser(
    id: Int = 1,
    name: String = "",
    email: String = ""
): User {
    return User(id, name, email)

But what if I need many of those users?

Glad you asked.

Create another function. Now you can generate as many unique Users as you want.

fun generateUsers(howMany: Int) = List(howMany){index->
        id = index,
        name = "Name #$index",
        email = "user_$"

We usually refer all test doubles as "mocks".

Keep it in mind