Dear K2 Admins,

I want to accept an input for a parameter when user create menu using K2 Items. And I'm going to use it from the template like this $tagheader = $this->item->params->get('tag_header'). Could you show me a way to do this? Thanks in advance.

This has to be implemented through a K2 Plugin.
You can find the example plugin here: getk2.org/extend/extensions/90-example-k2-plugin-for-developers