If you don't want to customize your NetSuite, you can add a mapping for NetSuite Record, then in the settings for that mapping, modify the Expression to your NetSuite URL.
Once that field is there, you just map it normally in Celigo and it flows into Salesforce without any issues.
We did try the “expression in Celigo” route as well (building the URL using {{internalid}}), and it works… but honestly it felt a bit patchy. The custom field approach has been much cleaner for us, especially when you have multiple flows.
In case it helps, here Celigo NetSuite we referred to when we were figuring it out (they explained the approach pretty clearly):