Go to POS configuration ===> Tick 'Linkly Payment'. Click on the linkly instance.

After that, you will redirect to the linkly instance list view. Open any one linkly instance.
In the linkly instance form veiw, 1st click on the 'Authentication' button to fill the details automatically.
To reset the refresh token, click on the 'Refresh Token' button.

In the payment methods form view, set the linkly details.

Click on the 'Send' button.

Generated receipt looks like this.

In the orders form view, you can see the Payment method in payments tab.