This is a duplicate of some other requests:
Insert internal link like Wordpress
Fast way to link to existing topic?
https://meta.discourse.org/t/how-about-mentioning-topic-feature/7350
You can kind of do something similar when composing by opening the header search, typing your query, using the down arrow ↓ to select the topic you want, and then hitting a to insert the link into the composer (totally recognize that it’s a power-user feature, and something in the hyperlink modal would be much more discoverable).