Create the workflow with the single action, Copy Item. Leave it to Current Item, and choose the existing list from the dropdown. Publish the workflow - leaving the workflow settings to only start manually.
Go to the list and manually launch the workflow on one of the items. When you get to the Nintex workflow launch screen, copy the entire link and go to Notepad.
Clean up and remove a lot of the URL like the Source parameter until you are left with something like this:
https://myCompany/sites/mySitename/_layouts/15/NintexWorkflow/StartWorkflow.aspx?List={1234d1e1-ccb3-4546-9676-123456789012}&ID=1&TemplateID={949ef476-4853-49a4-8c1a-605ddf23ed56}
Those are the only parts needed to launch the workflow: url stem, List parameter, ID of the item, and TemplateID which is the workflow. Remember, if you edit the workflow later, you will need to update the TemplateID part!!
On your form, add a Label control, go into the text area, choose Insert > Link from the ribbon bar and insert the link you created in the Address field, substituting the term {ItemProperty:ID} for the actual item number. For the hyperlink text, type Copy Item.
In my example above, the complete Address would be https://myCompany/sites/mySitename/_layouts/15/NintexWorkflow/StartWorkflow.aspx?List={1234d1e1-ccb3-4546-9676-123456789012}&ID={ItemProperty:ID}&TemplateID={949ef476-4853-49a4-8c1a-605ddf23ed56}
I did this as a simple link, as creating a button does not really work in a Responsive Form, and was not sure what kind of form you had.
I would recommend a rule to hide this link on not(isDisplayMode), as the copy item workflow will only copy the saved Item Properties (i.e. saved date from the form) and not any changes if they are in New or Edit mode.