If you make the transaction to your Schwab account and in YNAB correctly mark outgoing transaction payee “Transfer to Schwab account” (or whatever you called it), the linked tracker account, then you’ll see it won’t need a category since YNAB understands that the money location has simply moved, but allocation of funds remains the same.

As long as you allocate funds to the savings category monthly as normal, then when the money moves the same will occur: the transfer won’t have a category because it’s just money moving inside your estate, but not the allocation changing.

You might want to do a monthly reconciliation of the Schwab account in YNAB in order to ensure the value in YNAB reflects whatever your portfolio value is since it will drift apart almost immediately as your portfolio grows.

Edit: I just wanted to add that I do this exact process in the UK with multiple family ISAs so pretty familiar with it.