The steps of connecting to the profile, and of adding that you confirmed it with DNA, are different.
If the profile already exists, you can connect to it the same way you'd connect to any existing profile, by adding it as parent, spouse, or child of the other person.
You're absolutely correct that you should note the DNA confirmation. If the DNA evidence meets WikiTree's requirements (more in a second), go to the child's profile and select the "confirmed with DNA" button for the mother or father. If you do that, you'll need to add a "DNA confirmation citation", to explain the DNA evidence for the connection.
For example, one of my father's first cousins tested his DNA, and he and I match as expected. That confirms all the connection between me and that cousin (me to my father, my father to my grandfather, my grandfather to my greatgrandfather, my match's connection to his mother, and her connection to my great-grandfather). So, on each of the five profiles (mine, my father's, my grandfather's, my match's, and his mother's), I've selected the correct "confirmed with DNA" button, and added a DNA confirmation citation.
The help page for DNA confirmation (https://www.wikitree.com/wiki/Help:DNA_Confirmation) explains all the details.
Because working with DNA is complicated, not every match reported on Gedmatch counts--matches with small amounts of shared DNA may not even be real matches, and if they are, identifying the correct shared ancestor can be complicated. When does shared DNA count as evidence? That's still controversial and not completely clear. The Help page explains WikiTree's requirements.
Greg Clarke's DNA Confirmation Citation Maker app will help work through the details: https://www.wikitree.com/wiki/Help:DNA_Confirmation
If you still have questions (and I suspect you do!), please ask them.