Solved by following this guide: https://learn.microsoft.com/en-us/dynamics365/business-central/dev-itpro/developer/devenv-develop-custom-api

Step 3 did the trick, creating a page with a part:

page 50100 "API Car Brand"
{
    PageType = API;

    APIVersion = 'v1.0';
    APIPublisher = 'bctech';
    APIGroup = 'demo';

    EntityCaption = 'Car Brand';
    EntitySetCaption = 'Car Brands';
    EntityName = 'carBrand';
    EntitySetName = 'carBrands';

    ODataKeyFields = SystemId;
    SourceTable = "Car Brand";

    Extensible = false;
    DelayedInsert = true;

    layout
    {
        area(content)
        {
            repeater(Group)
            {
                field(id; Rec.SystemId)
                {
                    Caption = 'Id';
                    Editable = false;
                }

                field(name; Rec.Name)
                {
                    Caption = 'Name';
                }
                field(description; Rec.Description)
                {
                    Caption = 'Description';
                }
                field(country; Rec.Country)
                {
                    Caption = 'Country';
                }
            }

            part(carModels; "API Car Model")
            {
                Caption = 'Car Models';
                EntityName = 'carModel';
                EntitySetName = 'carModels';
                SubPageLink = "Brand Id" = Field(SystemId);
            }
        }
    }
}

And it's part:

page 50101 "API Car Model"
{
    PageType = API;

    APIVersion = 'v1.0';
    APIPublisher = 'bctech';
    APIGroup = 'demo';

    EntityCaption = 'Car Model';
    EntitySetCaption = 'Car Models';
    EntityName = 'carModel';
    EntitySetName = 'carModels';

    ODataKeyFields = SystemId;
    SourceTable = "Car Model";

    Extensible = false;
    DelayedInsert = true;

    layout
    {
        area(content)
        {
            repeater(Group)
            {
                field(id; Rec.SystemId)
                {
                    Caption = 'Id';
                    Editable = false;
                }
                field(name; Rec.Name)
                {
                    Caption = 'Name';
                }
                field(description; Rec.Description)
                {
                    Caption = 'Description';
                }
                field(brandId; Rec."Brand Id")
                {
                    Caption = 'Brand Id';
                }
                field(power; Rec.Power)
                {
                    Caption = 'Power';
                }
                field(fuelType; Rec."Fuel Type")
                {
                    Caption = 'Fuel Type';
                }
            }
        }
    }
}

Then go to Postman or your tool of preference and test this:

POST https://api.businesscentral.dynamics.com/v2.0/<environmentName>/api/bctech/demo/v1.0/companies(<company id>))/carBrands
{
    "name": "CARBRAND2",
    "description": "Car Brand 2",
    "country": "Germany",
    "carModels": [{
                    "name": "MODELA",
                    "description": "Model A",
                    "power": 0,
                    "fuelType": "Electric"
                },
                {
                    "name": "MODELB",
                    "description": "Model B",
                    "power": 0,
                    "fuelType": "Electric"
                }]
}