Like this:
const el = document.querySelector('#modal > div > div > div > form > div.Modal-body > div.Form-group.item-title > input')
el.addEventListener('keydown', (e) => {
if (e.target.value.length > 80)
{e.target.style.backgroundColor = 'red'}
else
{e.target.style.backgroundColor = 'unset'}
}
)
