In one of my project i have a multi-select custom field. I would like to filter all task that don’t contain specified selections from my custom field. Is it possible to add such feature?
Welcome to the Asana Community Forum @Patryk_Marek 
That currently works via the project filters.
Under “filters” within a project click “Add filter”
Then scroll down to locate the relevant multi-select custom field and you have the ability to pick which options should show and which not.
Add filters is not showing up on the project pages. Where do I find it?
Welcome, @julie57,
Have you added at least one task? It’s hidden until there’s at least one tab.
Thanks,
Larry