How to add an invoice link or save id to custome form.

[saket_bisht]

Hello, can any one help me regarding this problem.

 

I am working on a page where I need to submit a form like whats you need extra (where we have some options), now when user fills this form and submit then I need to linked an invoice ID with it, so admin can know that this form is related to this order and take suitable actions on that. thanks in advance.