Need Help passing field data from one content type to another
I don't have enough reputation to comment. I would approach this with an entity reference field, a block, and rules.

Your reservation content type should have an entity reference field for the 'customer' record.

A block would have the button to click. It goes on your customer record page. You might need a module like Rules Link  to trigger rules.

Set up rules so that when you click this button it:


creates a new entity (a node of type reservation)
populates the user reference field with the customer node you are looking at
saves the new node
opens the edit screen on the new node so more work can be done


It will take some playing around play around with rules to get it right.