Getting items from other doctype transaction

Hello All,

There is Add Items from button when creating new Purchase Order. That is a nice feature and I want to create custom field as a link type to Material Request.Then when i clicked get items from Material Request button the custom field should be populated with the source Material Request Name (eg,MREQ-0005) automatically.
and in the Item details table can i make some of the fields comes empty?
Can any one help me implementing this please.
Thank You.

Purchase Order already has option to pull items from Material Request, hence no need to add custom link or link field for it.

In the Item master (or any doctype), for each field, defalt value is defined from the customize form. You can edit default value for the field from Customize Form.