<input type="text" name="amount[0]">
<input type="text" name="amount[1]">
<input type="text" name="amount[2]">
<input type="text" name="amount[3]">

$browser->visit('/create')
    ->type('amount[0]', '100')
    ->type('amount[1]', '100')
    ->type('amount[2]', '100')
    ->type('amount[3]', '100');

$browser->visit('/create')
    ->type('amount[]', '100');