can use seatbelt directly.

however, some seat provides 'isofix mount" adapter.

Seat belt car seat is for front facing, if your children is still baby that require rear facing, it not gonna work.

Try slip your hand behind the seat see is there any hook? Or take of the bottom seat to verify more accurately for the isofix.

I have been using Graco extend2fit for my kid. They come with isofix latch system which is a lot more convenient vs the rigid isofix base system in some car seats. The good thing with this carseat is that you can also use it with seatbelt both for rear and forward facing. Can use from infant like I have been using for my kids. currently my 4 y/o are sitting in it.

My advice is buy this kind of car seat with latch isofix system as its normally can also be used with a seatbelt. And in case you upgrade your car with an isofix in the future you can also use the isofix function. And the installation using isofix latch is much2 more easier IMO vs the isofix base system based on my personal experience.

Lazada/shopee got sell the metal for retrofit however, how safe is it u ownself decide.

Anyhow there are still many designs for non-isofix, also i believe isofix models also can use seatbelt. Just check with seller before buy.

1. How old is your child, and what's your child's weight?
2. Why must so adamant want ISOFIX?

1. Firstly congratulations.
Secondly, not necessary must get those with ISOFIX features. ISOFIX features is something the child cradle or seat has that would easier "lock into" the car's seats. It's something "nice" to have, but not mandatory. You can still get 1 with ISOFIX features, use it in your partner's car, and still use it in your own car.
A lot of baby outlets have these car seat demo which you can ask the staff to demo to you on how you can fasten/secure your baby carrier onto the seat.

What you need at this moment is just to SECURE the baby carrier onto the seat, with the car's normal seat belts also can.
As long as the carrier is, or can be secured properly without it being able to move around on its own, then it should be OK.
Also, no person in their right mind would be driving like a maniac with a newborn baby in the car, right? 

isofix more convenient when u wanna install/uninstall the seat.. some design u have to tie the belt in a very convoluted way to secure it.