proxy – how to sniff android protected app Http request

I am going to capture one app Http request to get Login API .

Not My app . only school homework .

i am using fiddler .

But when write Username and password . and pressing sing in . app working
But in fiddler i can not find login http request .

i tried with charles proxy too , same problem

also i did some more app , but getting this problem in some apps

How to solve it ?