Loading more data in API - api

Question

I am new to programming and I am working on a personal project using html, css, and vanilla JavaScript I have used a reddit highlight API to get NBA highlights. In the initial fetch API I have the limit set to 20 items to load on the page. I am trying to load the next 20 items from the endpoint on a load more button at the bottom of the page.
I know I have to use &=after to load the next set, but when I code it to the load more function it only give me the next 20 not 40 total.
Here is my code :
const loadMore = document.querySelector('button');
//Fetch API
const fetchData = () => {
const redditHighlight = 'https://www.reddit.com/r/nba/search.json?q=flair%3AHighlight&restrict_sr=on&sort=new&t=all&limit=20'
fetch(redditHighlight)
.then(response => {
if (!response.ok) {
throw Error('ERROR');
}
return response.json()
})
.then(data => {
console.log(data.data);
const html = data.data.children.map((item) => {
return `
${item.data.title}
`;
}).join("");
console.log(html);
document.querySelector('#app').insertAdjacentHTML("afterbegin", html);
})
.catch(error => {
console.log(error);
});
}
fetchData();
loadMore.addEventListener('click', () => {
});