Multi-Select Detail

Hi Everyone.

I have created a detail in my Order's sceen in Classic UI. I want to now enable Multi-select in that Detail. Similar to something like this:

https://customerfx.com/article/programmatically-enabling-multi-select-a…

Now, I am new to creatio. I just need to know where to put this code in the Source code? And Can I use this selection of multiple contacts to trigger a message to those that are selected only from that grid?