Hi Gudu,
there are multiple ways you could achieve this. You could use a workflow, a flow, or even a simple business rule.
As using a business rule would be the easiest for a beginner, I'd suggest you to take a look into them.
You could do something like
After insert business rule
table: sc_req_item OR sc_task table (depending what you want to achieve)
condition the RITM's item is your current catalog item from portal
You can also script business rules - what you would need is an easy insert glide record query.
There is a very useful cheatsheet even more experienced devs use out of habit. I'd advise you to check out the snippet for the insert and update gliderecords
https://www.servicenowguru.com/scripting/gliderecord-query-cheat-sheet/
