Ask the host what you should bring along
This clearly carries the message that you will bring the item you will bring. And this clearly shows that you will be coming to bring the item you have offered to bring.
Don't show up empty-handed. Ask the host "what do you need?" and bring that.
Something token like a bag of chips/crisps, or if you cook make something like scones or biscuits/cookies.
If the host says "don't bring anything" then you can either listen or show up with a bottle of wine or soft-drink or juice, because that's always useful.
Either way, you had a conversation with the host and they now know you're coming.
Expansion, I personally do this with my sister, she used to say "just bring yourself", until the day I said "tell me what you need or I'll bring Haggis" so I turned up with canned Haggis. Since that time, she now says what they need.
Either way, she now absolutely knows we're intending on coming. This is my personal experience, in western society.