How to target a random generated number?

I have a script that randomly generates a room id when joined. I want to copy that ID with a click of a button. It would be easy job with a input element, however, I don't know how to even target that random ID to edit and manipulate it. How could I do that?

function makeid(length) {
    var result           = '';
    var characters       = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789';
    var charactersLength = characters.length;
    for ( var i = 0; i < length; i++ ) {
       result += characters.charAt(Math.floor(Math.random() * charactersLength));
    }
    return result;
 }
 
 var roomUrl = document.createElement("room_id_div");
roomUrl.innerText = `Room id: ${room}`
roomID.append(roomUrl)

<div id='roomId'></div>