Assistant billing in Medilink is basically the same as standard billing.
The assistant needs to be an actual provider in Medilink - just like any other provider.
When performing the actual billing they need two extra things at the front of the description (important!), the surgeon/principal provider number and the surgical items. For example:
51303* Surgeon=2107501Y Item=45671* Assistance $127.10
Also the assistant item (e.g. 51303*) needs to be in your item fees list. Depending on the item you use the amount is typically 1/5th the surgical item so it will likely change (i.e. you may have to manually change the amount each time you bill and/or create multiple versions of that assistant item with different descriptions). Please refer to the MBS guide if in doubt.