fetch with the whole url work but not when i’m setting proxy, why ? should i create constant instead?

If I try to get datas from my local server with the whole url, it works:

fetch("http://192.168.xxx.xxx:3000/members")
        .then((response) => {
            return response.json();
        })
        .then((myJson) => {
            console.log(myJson);
        })

output:

{"members": ["Members1", "Member2", "Member3"]}

but if I’m setting a proxy in my package.json, it doesn’t:

"proxy":"http://192.168.xxx.xxx:3000/",

fetch("/members")
        .then((response) => {
            return response.json();
        })
        .then((myJson) => {
            console.log(myJson);
        })

output:

TypeError: Network request failed

I have read somewhere that it’s not possible to set up a proxy in react native ?
So does the good way to do is to set a constant that I call each time?

For add some details:
I use expo and start it like this: npx expo start --dev-client --tunnel