You have to raise your forum trust level to post external links.
from New Forum Guide - Syntax and Features
Trust Levels
Trust levels are a participation system on the forums. The more you post, like, reply, read, and visit the forums, the more you work your way to a higher trust level. Increased Trust levels grant additional privileges. Everyone who is new starts off at Trust Level 0 but will quickly climb by reading and visiting the forums within their first few days.
TRUST LEVEL 0
Trust level 0 users cannot:
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Flag posts
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Post more than 3 topics
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Post more than 10 replies
TRUST LEVEL 1
Get to trust level 1 by…
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Entering at least 5 topics
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Reading at least 30 posts
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Spend a total of 10 minutes reading posts
Users at trust level 1 can…
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Post unlimited topics and replies
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Edit wiki posts (more on Wiki Posts Below)
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Flag posts
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Create links and video embeds to trusted websites like Blizzard websites, YouTube, or Twitch
TRUST LEVEL 2
Get to trust level 2 by…
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Visiting at least 15 days, not sequentially
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Casting at least 1 like
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Receiving at least 1 like
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Replying to at least 3 different topics
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Entering at least 20 topics
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Reading at least 100 posts
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Spend a total of 60 minutes reading posts
Users at trust level 2 can…
- Have an increased “like” count.
TRUST LEVEL 3
Get to trust level 3 by…
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Must have visited at least 50% of last 100 days
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Must have replied to at least 10 different topics
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Of topics created in the last 100 days, must have viewed 500 topics
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Of posts created in the last 100 days, must have read 20,000 posts (both starting posts and replies)
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Must have received 20 likes, and given 30 likes.
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Must not have received more than 5 spam or offensive flags (with unique posts and unique users for each, confirmed by a moderator)
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Must not have been suspended (including forum suspensions or in-game account actions of any Blizzard game)
Trust Level 3 users can
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Create Wiki Posts (more on that below)
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Post animated gifs, images, compatible video embeds, and links to any website