As mentioned here How to add link to topic title? you can’t add a link after the topic was created, but I have a workaround that may work for you:
Edit the whole topic, not only the tile, by clicking the button at the bottom of the topic:
Then you have to remove the content of the topic (copy and paste it somewhere safe), replace the title with the link. You will see how the link is loaded in the title, and the onebox appears in the topic composer. After that, you can paste the topic content again.
It’s basically “creating” the topic again, since links in titles can only be added during creation.