Where do I find usage instructions for these hooks: membership_add_subscription and membership_payme

Hi,

I need to write custom code that uses the following Membership hooks:

membership_add_subscription
membership_payment_processed

However, I cannot find any place with information on how to use them. Please point me to the instructions?

Basically, this is what I am trying to achieve (got this advice from @Rheinard) on the https://premium.wpmudev.org/forums/topic/desperately-need-ecommerce-tracking-on-membership-plugin-happy-to-pay-for-it#post-789360 thread:

"...make a shortcode that you can put on your landing page. This shortcode would be responsible for capturing the referral URL in a session variable or a cookie. Hook the 'membership_payment_processed' action. By hooking this action you can add additional data to the cookie that will be processed when 'membership_add_subscription' fires. Use the 'membership_add_subscription' action to hook a function that will pull the relevant bits of information from the action, but also the referral URL from the cookie."

I am lost as to how to 'use the 'membership_add_subscription' to hook a function'... Pls let me know where I can find info on using these Membership hooks.

Thank you very much and Merry Christmas!

David