So many questions - is this an existing profile on Wikitree? If it is, how do you want to add it? As a father, mother, sister, brother, child?
If it is an existing profile - let's call it Basque-2000 and let's assume for the moment that we've discovered that this is the father of Basque-1000, then you would:
Go to Basque-1000's profile and click the Edit Tab
Go to the Father on the right hand side and click Add Father
Right at the very top, it says
A.) To set someone who has an existing profile on WikiTree as the father of <Basque-1000's name here> enter their WikiTree ID here:
So enter Basque-2000 in this block, scroll down, click the Add Father button.
And then Basque-2000 would be set as the father for Basque-1000
There are all kinds of restrictions you could run into (what if Basque-1000 already has a father, what if Basque-2000 is restricted in some way, what if they are both pre-1700 or pre-1500 profiles, etc. etc.) but this is the basic way to add one profile to another.