Marco, something like this using the javascript replace function on a field named “description” where the n flag is for new lines and the g flag signifies all occurrences, not just the first one. If this element is connected to a dataset field, you will want to disconnect it and let this code do the assigning.
export function repeater1_itemReady($item, itemData, index) {
let replaceText = itemData.description.replace(/\n/g, " ");
$item("#textbox1").value = replaceText;
}