App Monetization Guidelines

There's no code to add.  If you want something within your app, you need to do it yourself.  But then if you do, you need to indicate in the app description and details that a payment is required.

Sounds like someone just wants to make a donation to you, in which case, just point them to your paypal account.

Thanks.

But it's looks confused by user a banner that it's says "payment required" and whe you download nothing is requested. I am receiving some mails requesting intructtions for payment.

There is a checkbox in your app details page.

You only need to select yes if your app requests or requires payment. 

If your stuff is entirely free, you can select no. 

And if you don't require payment, you can leave the box unchecked, and in the description have something like

"If you like the app, and want to buy me a cold drink, you can do so by way of (your paypal account)"

Or a hot drink, if you prefer coffee. ;)