Well, you're going to get all kinds of answers for this one, but I'll give you my recommendations.
Since you're not the biological parent, nor are you the adoptive parent, I'm not certain you would add them as your child, but that's up to you. You could add them as a separate profile and then just put a link to them in your biography (I fostered WIKITREE-ID from xx date to xx date).
But to add them as a separate profile, you'd just go to "Add" and "New Person" and enter them in.
If you want to enter them in as your child, you would just enter them in as any other child, but you might mark them as non-biological in the settings so that it's clear that they're not your biological child, but again, that's up to you.