Hi KenPhoto, thanks for joining the Community.
You can access this on QuickBooks Self Employed by going to the Transactions menu (Take me there) > Find the transaction you want to split > Select Split from the Type column. This opens the split transaction window, where you can select Percentage or Amount from the Split by ▼ dropdown. If you don't see this option, try loading QuickBooks in a different web browser or incognito window.