You need to supply Form::select with companies as an array('id'=>'name'):
// Controller, repo or wherever you want it:
$companies = Company::lists('company_name','id');
// $companies passed to the view, then in the create view:
{{ Form::select('company_id', $companies, null, $options) }}
// edit view:
{{ Form::model($customer, array('route' => array('YourCustomerUpdateRoute', $customer->id))) }}
...
{{ Form::select('company_id', $companies, null, $options) }}
// form model binding autopopulates the form, so correct option will be selected
After submitting the form validate the input, check if provided company_id exists on the companies table and save the customer, that's all.