How to send an email with variables using Mailjet API v3.1?

Question

I just want to know why when I send variables using a template, the Mailjet API doesn't replace them, even the fakeFirstname doesn't show.

I just see {{var:FirstName:"fakeFirstName"}}

Here is my payload :

{
    "Messages": [
        {
            "From": {
                "Email": "senderEmail",
                "Name": "senderName"
            },
            "To": [
                {
                    "Email": "userEmail",
                    "Name": "userName"
                }
            ],
            "Subject": "subject",
            "TextPart": "",
            "HTMLPart": "",
            "TemplateID": TemplateId,
            "TemplateLanguage": true,
            "Variables": {
                "FirstName": "otherName"
            }
        }
    ]
    
}

Thanks for the helpers.