My experience is that the linkage is done by the tournament director along with the ICC support team. I didn’t try to do this in USCF site. For example, CCA has its own registration page, https://register.chessclub.com/cca, which will ask you to provide your USCF ID in the second step. Once you register for a tournament, I typically get an automatic email to ask you to send your USCF ID and ICC username to the TD so that they can confirm the linkage or setup the linkage for you.
The ICC support contact can also help on this issue. They can confirm if the linkage is established in their system.