Hi Djurcinovska1, 

Thank you for reaching out on this. 

If you have created the project already, you can go back to the existing invoice and click on the drop down box for customer on the top left. You should be able to see the project you have created under the customer name. By selecting said project as the customer on the invoice, said invoice will be added to the project. 

Hope this helps and please let us know if you have any questions.