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HTML

<input type="text" placeholder="Place given Access Key" id="accesskeynum" name="accesskeynum" value="accesskeynum">    
    
<a href="immobilization_actions.php?action=immobilizerx" style="margin-right:50px;"><img src='style/Immobilize.png' height="75" width="75" /></a>

JavaScript

var ele = document.querySelector("a");
var input =  document.querySelector("#accesskeynum");
ele.href += "&id="+ input.value;

input.addEventListener("change", function() {
    ele.href = ele.href.replace(/&id=[^"']*/, "");
    ele.href += "&id="+ this.value;
});

ele.addEventListener("mouseenter", function() {
    this.src='style/Immobilize.png';
});
ele.addEventListener("mouseleave", function() {
    this.src='style/Immobilize.png';
});