Simply check if the input startsWith what you're looking for..

let input = document.getElementById("input").value;
let text = input.startsWith("https://") ? input : "https://" + input;

As noted in the comments, the conditional ternary operator has been used here, you can find out more about it here: Conditional (ternary) operator

One approach is:

// here we cache the value of the <input> element:
let value = document.getElementById("input").value,
    // we define the protocol we're looking for:
    protocol = 'https://',
    // we then assign the text, using a template-literal string,
    // first we use a conditional operator:
    // which returns a Boolean (true/false), if the
    // String we test starts with the supplied String
    // ('protocol'); if it does we return an empty string
    // and if not we return the protocol variable; and then
    // that's concatenated with the existing 'value'
    // variable:
    text = `${value.startsWith(protocol) ? '' : protocol}${value}`;

References:

• Conditional operator.
• String.prototype.startsWith().
• Template literals.

Try to use includes function to check if the string contains another string.

document.addEventListener("keydown", function (event) {
  if (event.keyCode === 13) {
    let text = document.getElementById("input").value;
    let src = text.includes("https://") ? text : "https://"+text;
    document.getElementById("iframe").src = src;
    document.getElementById("iframe").height = "100%";
    document.getElementById("iframe").width = "100%";
    document.getElementById("content").innerHTML = '';
  }
});