Make dropdown menu required

I am building some React form, just build dropdown menu but i don't know how to make it required. i use useState to show selected property to dropdown afther user click, before user clicks, it's default value is "team",when clicks i change state using changePosition() function.

const [teamsDropdown, setTeamsDropdown] = useState("team")

const changePosition = (e) => {
    e.preventDefault()
    setPositionDropdown(e.target.getAttribute('a-key'))
}  

<div className="dropdown">
    <button className="dropbtn" onClick={handleStyle}>
        {teamsDropdown} <img className="invector" src={vector}/>
    </button>
    <div className={style}>
        {teams.map(team => <a href="#" onClick={changeTeam} a-key={team.name} key={team.id}>{team.name}</a>)}
    </div>
</div>