Getting the name of a held item

how would I go about getting the name of a held item, I have currently got it to use the item itself with SelectedItem but I want to use the custom name. 
ex. foo:bar contains:
$say $(custom_name)
while holding a sword named "hello" and using the command:
/function foo:bar with entity @s custom_name
expected result:
A message in chat saying hello