<select name="" id="selection1">
<option value="1">Option1</option>
<option value="2">Option2</option>
<option value="3">Option3</option>
<option value="4">Option4</option>
</select>
<select name="" id="selection2">
<option value="5">Option1</option>
<option value="6">Option2</option>
<option value="7">Option3</option>
<option value="8">Option4</option>
</select>
<button><a href="#" id="linkBtn">Go to link</a></button>

​

JS

const myButton = document.getElementById('linkBtn')
document.getElementById('selection2').addEventListener('change', (e) => {
let selection = parseInt(e.target.value);
switch(selection) {
case 5:
myButton.href = 'https://www.reddit.com'
break;
case 6:
myButton.href = 'https://www.google.com'
break;
case 7:
myButton.href = 'https://www.sexygrannies.com'
break;
case 8:
myButton.href = 'https://www.bbc.co.uk'
break;
}
})