Visit the website at https://console.developers.google.com/.

Sign in using your Google account. If you don't have a Google Developers account yet, you might need to create one.

Locate and press the "Select a project" button located in the top toolbar. If you have existing projects, the button will show the name of one of your projects.

After the pop-up window appears, select the "New project" button situated in the upper right corner.

Provide a name for your project, such as your marketplace's name, and then press the "Create" button.

Your project has been successfully created. The next step is to configure it.

Navigate to the "OAuth consent screen" page. You can access this page from various links on the interface, but the simplest route is through the menu located on the left side of the screen.

When you're on that page, choose "External" as the User type and then click on "Create."

On the following page, you can omit filling out most of the fields, but it's essential to include at least the app's name & User support email & developer contact information.

Afterward, click on the "Save" button.

You can skip the Scope and Test users section. In the Summary step, simply click "Back to Dashboard."

Next, access the "Credentials" page from the left sidebar.

Click "+ Create credentials" from the top of the page.

Select "OAuth Client ID"

Select the application type "Web application"

Feel free to choose any name you prefer.

Under "Authorized JavaScript origins," input your complete marketplace URL with "https://" and potentially "www" or the subdomain, without including the trailing slash. For example, it should appear as follows: "https://www.example.com" or "https://example.qoreups..com".

Within the "Authorized redirect URIs" section, include your marketplace address followed by "https://www.example.com/confirmation" or "https://example.qoreups.com/confirmation".

Click "Create".