How to retrieve data from the subgrid entity to the current entity

Hi people,

I have an entity (courses) which has a subgrid to add participants (another enitity),

now i save the form and click the subgrid and add new participants,

i fill the participants entity, now i want to pick a name field from participants and save it in the current entity's (courses) field.

So i just need to know how to retrieve the data from the subgrid, and on which event i should trigger the function