TikTok only allows users with at least 1,000 followers and a Business account to add a clickable linkinbio. If you don't have these requirements, check out these alternatives.
TL;DR: Video at the bottom of the page
Step 1
Head over to your Lnks page, and tap on the Copy button located at the top-right of your page, as shown below:
Step 2
After you copied the link, head over to your TikTok App, access your profile, and click on Edit as shown below
Step 3
Now that you're within the Edit Profile section, click on Links to access the list of available links.
Step 4
Finally, click on "Website" and add paste the previously copied Lnk.Bio URL
All done!!
Video guide
Here's a full video showing the process