x-select-enable-primary

x-select-enable-primary is a variable defined in `select.el'.
Its value is

nil

• This variable is an alias for `select-enable-primary'.
• This variable is obsolete since 25.1;
  use `select-enable-primary' instead.

Documentation:
Non-nil means cutting and pasting uses the primary selection
The existence of a primary selection depends on the underlying GUI you use.
E.g. it doesn't exist under MS-Windows.

You can customize this variable.