I would use @kuovonne’s Prefilled Forms App to help you troubleshoot what’s wrong with your URL:

Also, note that your prefill option for a linked record field has to exactly match a pre-existing record in the linked record table.

Welcome to the Airtable community!

Your prefill is not working because you have multiple records with the same value in the primary field. In this situation, Airtable cannot tell which of the four records you want to link to, so it does not link to any of them.

Solution 1: Change your primary field values so that every record is unique.

Solution 2: Prefill using the record id of the desired record, instead of the primary field value.

Hello,

In my case even though I switched to the record id, instead of the value - the field still doesn't populate. 

Same issue here.

Prefill by Linked Records doesn't work for me too.

Confirmed.

Maybe something broke when they moved forms into interfaces. The link to the form also changed and the docs are not up to date anymore.

Temporary solution: I worked around it by storing the record ID in a separate field and made an automation fill my original field.

Update: it does work, but only sometimes as long as you are testing it in the same browser where you do the designing (Edge in my case). When I use the exact same link in a different browser where I'm not logged in it works fine.

Maybe a strange buffering / record caching issue? Or an Edge thing?

Confirmed here as well

@Tuur is correct. If using the link in a separate browser from the designing browser, it works fine. So the form prefill works for your users, but not you (at least initially). I have tested it with other users and the form prefill works fine for linked records.