How can I use foreignId in Laravel with camelCase?

I have a table User, which has an id. In another table (AdminUser), I want to reference it, but with camelCase convention.

However, Laravel doesn’t convert that and/or recognize the user table, so it returns always null when asking for the user from the other table.

I am experimenting with something, so the table layout User and AdminUser might be weird, but that is not the point of the question.

Here is the AdminUser table:

Schema::create('AdminUser', function (Blueprint $table) {
    $table->id();
    $table->foreign('userId');
    $table->integer('pin');
    $table->string('passcode');
    $table->timestamp('updatedAt');
    $table->timestamp('createdAt');
});

Here is the User table:

Schema::create('User', function (Blueprint $table) {
    $table->id();
    $table->string('username');
    $table->string('password');
    $table->enum('role', ['ADMIN', 'USER'])->default('USER');
    $table->string('loginToken')->nullable();
    $table->rememberToken()->nullable();
    $table->renameColumn('remember_token', 'rememberToken');
    $table->timestamp('createdAt');
    $table->timestamp('updatedAt');
});

If I reference it with user_id in the AdminUser table, it is working properly, but I would like to find out how to use camelCase foreign keys.