Achievement IDs can be seen on the HTML code of the character page when choosing the Achievements that will be shown on your page. The HTML code can be seen by using the browser’s “Inspect” feature, and each checkbox has its value as the corresponding achievement’s ID
So, log in Tibia.com, go to any character on your account and click "Edit" as if you were going to change the comment or displayed achievements. Then, right click the checkbox of the achievement you want to check and select "Inspect" (or a similar option, varies with browser). You'll see some HTML code with the ID like this:
<input type="checkbox" name="achievementlist[]" value="187" checked="checked"></td>
Source: https://tibiald.com/2021/05/03/achievement-ids/