example.js

const firebaseApiKey = pm.environment.get("firebaseApiKey")
const firebaseTestUserEmail = pm.environment.get("firebaseTestUserEmail")
const firebaseTestUserPassword = pm.environment.get("firebaseTestUserPassword")

const options = {
    url: `https://identitytoolkit.googleapis.com/v1/accounts:signInWithPassword?key=${firebaseApiKey}`,
    method: 'POST',
    header: {
        'Accept': '*/*',
        'Content-Type': 'application/json',
    },
    body: {
        mode: 'raw',
        raw: JSON.stringify({
            "email": firebaseTestUserEmail,
            "password": firebaseTestUserPassword,
            "returnSecureToken": true
        })
    }
};

pm.sendRequest(options, function (err, res) {
    const respData = res.json();

    pm.environment.set("firebaseIdToken", respData.idToken);
    pm.environment.set("firebaseUID", respData.localId);
});