I keep getting “Unknown custom field” error trying to add a contact with custom field to SimpleTexting

Here’s my code:

app.post('/api/outgoing-from-ac', async (req, res, next) => {
    try {
        const {contact} = req.body;
        const {email, first_name, last_name, phone, tags} = contact;

        let response = await axios({
            method: 'post',
            url: 'https://app2.simpletexting.com/v1/group/contact/add',
            json: true,
            data: querystring.stringify({
                token: config.simple_texting.token,
                group: 'Registrants',
                phone,
                firstName: first_name,
                lastName: last_name,
                email,
                custom_tags: tags, // <--- This does not work!
            }),
            headers: {
                'content-type': 'application/x-www-form-urlencoded',
            },
        });

        console.log(response.data);

        res.json({
            status: 'OK',
        });
    } catch (error) {
        console.log(error);
        res.json({
            success: false,
            message: 'Error!',
        });
    }
});

So I’m trying to add contacts to a list in SimpleTexting using their API, along with tags with the other data, I removed all the unnecessary codes since everything else is working just fine.
When I try to add a contact without custom_tags, it adds, but when I use the custom_tags, it gives out the error { code: -611, message: 'Unknown custom field' }
This is what their documentation reads:

customField 
string
Any custom field of a contact, where customField` must be replaced with the actual custom field’s parameter.

For example, if you have a ‘Street address’ custom field, you would use the parameter ‘street_address’.

I tested customField, I tested object inside customField, I tested custom_tags, I tested tags, nothing is working. What is the issue here? How should it be coded?