This article explains how to create a Google custom App and obtain the right permissions to use it.
How to create a new Google custom APP
STEP 1
Log in to your Google account and go to API & Services then click Create to create a new project.

INFO
Use Select button to choose an existing project.
STEP 2
Write the name of your project in the appropriate field and click Create

STEP 3
From Navigation Menu go to API&Services then Credentials

STEP 4
Make sure you are in your project as shown on the top left, then click Credentials

and choose as type OAuth client ID

STEP 5
Before you can create your credentials, you will be asked to configure the consent screen:

The consent screen will be shown to users whenever you request access to their private data using your client ID.

Please replace https://splash.mydomain.com with your Splash Page url.
Click on Save .
STEP 6
After the previous step you will be redirected to the creation of your credentials. Be sure to choose Web application as Application type and fill in the Authorized redirect URIs fields.

The format of your authorized redirect URIs shoud be:
https://splash.MYDOMAIN.com/index.php?action=oauth_google_callback
Please change MYDOMAIN with your current setting, according to this article:
Creating CNAMES for your Splash Page, Dashboard and Splash Editor
Click Create to proceed.
STEP 7
Your App Id and Client Secret will be displayed:

STEP 8
Write down the CLIENT ID and the CLIENT SECRET, the two codes must be set in your White Label Splash Editor.
Related Article:
How to configure the Tanaza Splash Page to work with your custom app