Hi,
Set trigger 'When record matches condition' Source not empty AND Story not empty AND Content type is empty
Create at least one record, fill Source and Story with anything, to match condition, test the trigger by 'Suggested record' or 'Choose record' 

Set action 'Update record', choose table, choose record ID 

Then set field(s) to update . Fill with static name of a record which you want to link 

Test the step, enable automation. That's all
Records already matching at the moment you enabled automation, will not trigger it.  
You can clean Source, for example and type there text again (or Ctrl+Z to Undo cleaning). This will re-run automation for the record matching condition. 

We are half way there but does not solve full problem. Please watch quick video for reference on what I mean.

https://www.loom.com/share/90b6c147afd640509ec8363e26bf8568?sid=2423394b-1d58-46d0-903b-fd1f278b6d3b

I got it. Usually it should work, but sometimes pasting a value into a link field could cause problems when value contains comma or ampersand.
I think it should work if you add it as "Reels & Shorts" instead of Reels & Shorts
or, it should 100% work if you add it as Record_ID. Just add formula with Record_ID() to the 'Video Dimentions' table and paste recXXXXXXXXX value in automation  instead of Reels & Shorts

How do you get the exact record id from the video dimension table ?