Minibuffer - emacs-helm/helm GitHub Wiki

While in a Helm session, you can delete minibuffer all contents with C-k (helm-delete-minibuffer-contents).

When the customizable variable helm-delete-minibuffer-contents-from-point is non-nil, helm-delete-minibuffer-contents deletes from point to the end of the minibuffer. When using a prefix arg this behavior is reverted. When using a prefix arg when at end of minibuffer, delete all but also preselect current candidate if it is not beyond helm-candidate-number-limit.

You can use shift selection to mark text and copy/kill it.