Please follow some below steps then you will get your firebase url for poll / feedback plugin..

Step 1 : Open https://firebase.google.com/
Step 2: After login click on "go to Console"
Step 3 : Add New Project
Step 4: Enter Project Name, then accept the terms and condition and click "Create Project" button.
Step 5 : Click on "Develop" menu in left sidebar
Step 6 : Click on "Database" Under the "Develop" Menu
Step 7 : Now Click on "Create Database" button
step 8 : Select "Start in Locked Mode" option and click on "Enable" button as per screenshot
step 9 : Select "Realtime Database" from top bar in the nearby "Database" title as per screenshot.
Step 10 : Click on "rules" tab and paste below code
{
"rules": {
".read": true,
".write": true
}
}
then click on publish button.
Step 11 : Just copy that it should display like : https://your-project-name.fire...
Step 12 : Put https://your-project-name.fire... this link in wordpress plugin as per screenshot fro plugin configuration