'On Save' trigger

'On Save' trigger

Hi, 

How can I trigger a function after clicking save?

Currently I use a button, but it's not ideal. 
The function redirects to a Creator app that alters a value, then inputs and redirects back into the same CRM record.

While i'm at it, why can't an action be triggered 'On User Input' like in Creator?

This would be a really useful feature to have in CRM.

Thanks!