To assign a portion of tips to an assistant, you first need to create an "Assistant Service" (or something similar) if it has not already been created. Make the price of this service $0 so it does not affect the total for the client.
In the checkout, add the "Assistant Service" to the checkout and assign the correct assistant that should receive the tip:
Complete the checkout, and then re-open the sale:
Select "Custom Split" and adjust the tip amounts accordingly: