Unknown field "assigned to"

Good morning,

I'm still working on the formula around the "Assigned to" and this field really giving me trouble.

So i want a field to indicate "1" if the case is assigned to X.
I wrote : 

ifElse(equal(related($assigned_user_link,"assigned_user_name"),"Yves Bourgarel"),"1","2")

But it is telling me "related: Unknown Field : assigned_user_name"

Whe i inspect the page, assigned_user_name definitaly seems to be the name of the field. I'm not sure if i should have a "related" function before, but even without it, i have the same problem.

Any ideas ?

thank you