If you are using the Topic Prefix Extension and want the topic prefix to be mandatory, here is an easy solution supported by most browsers.
Open:
/ext/phpbb/topicprefixes/styles/all/template/event/ext/phpbb/topicprefixes/styles/all/template/eventFind:
<select name="topic_prefix" id="topic_prefix">Replace with:
<select name="topic_prefix" id="topic_prefix" required>Okay, so you only want it to be mandatory in specific forum(s)?
You need to know the forum ID - you can see it in the link when visiting the specific forum.
In this example we want to exclude forum ID 6 from requering the topic prefix.
Then the replace would be:
<select name="topic_prefix" id="topic_prefix"{% if FORUM_ID eq 6 %} required{% endif %}>If more forums should be excluded the replace could be:
<select name="topic_prefix" id="topic_prefix"{% if FORUM_ID in [2,4,11,25] %} required{% endif %}>Save, upload and purge cache.