Form Update Database Model Record's Relationship

I want to know how to up date a model's relationship using a front end form. I looked at the documents and found:

Alternatively you may set the relationship using the primary key, this is useful when working with HTML forms.

// Assign to author with ID of 3
$post->author = 3;

// Assign comments with IDs of 1, 2 and 3
$post->comments = [1, 2, 3];

$post->save();

This is my code: where I am getting ID as a value but it doesn't seem to affect the relationship field. Help would be great thanks!

$project->client = Input::get('client');

Image of the relationship in the builder.

via Chebli Mohamed