[Typo3] Relation in the TCA?

[Boris]

I have 3 tables:
- tab_product
- tab_action
- tab_action_mm (Verknuepfungstabelle)

The tables "tab_action_mm" consists of these fields:
- uid_local
- uid_foreign
- action_price

To every product a price is deposited. Now one can put on actions which 
count to certain products and then the price of the concerning product 
changes.

I know how the relation basically works (TCA). Now I have the following 
question:
How to configure the inputform in the backend in the way like it creates 
the option to enter an action-price when I assign products to an action?