To add a retro border to Tik Tok, you can follow these steps:

1. Open the Tik Tok app and go to the edit page.

2. Select or record a video that you want to add a retro border to.

3. Click the edit button to find the "Border" option.

4. In the border options, you can choose your favorite retro border effect. Sometimes you may need to swipe the screen to see more effects.

5. After selecting a suitable border, you can click the "Save" or "Apply" button.

6. Then make other edits to the video in the edit page, such as adding filters, music, etc.

7. When you are done, click the "Next" button and you can choose to post it to Tik Tok or save it to the album. In this way, your video will have a retro border. Remember to explore different effects in the border options to find the retro border that suits you best.