You probably first need to understand how the portal works with regards to users, and this may influence how you do things.
Each logged in Portal user will have a Contact record and this is the portal user (e.g. if you use Azure AD/Entra you will end up with a Contact record in Dataverse that is associated to an External Identity record that holds the linkage to the person in AD/Entra). By default your Entra/AD users will not initially exist in Dataverse as Contact records (depending on your authentication settings, you will either need to create them and invite them to register, or let the system create them when they register - there are pros and cons with both approaches).
Key thing to understand is a Portal User = a Contact record, not a System User (Dataverse / Model Driven Apps use System Users for ownership of records and created/modified by) this distinction is important as the portal does not recognize Dataverse System Users or Teams.
You are saying the portal user will select themselves from a list, in the context of a Contact record this would not be necessary (unless they are also going to be able to select other people) as when the other record is created you can automatically populate it with the logged in users Contact record.
However, the follow on question is where else are you wanting to use the data and does that really need a System User rather than just a Contact e.g. if the same user is to log into a Model Driven app and your expectation is that the record is Owned by that user (note, this is not an out of the box feature and would require some customisation).
As an example: If a portal user creates a record that record will be Owned by the Portal (it has its own system user) not the persons dataverse system user (if they are one), the Created By/Modified By will also be the Portals System User - if the portal user needs to see only their own records on the Portal you would link the record to the Portal User by having a Lookup field to Contact on the record and populate that lookup with the portal users Contact record.
