Hello, @Monika1983. 

In QuickBooks Online (QBO), an e-signature isn't available yet. However, we can use the QBO mobile app to accomplish the task. 

Here's how: 

For Android, we can follow these steps: 

1. From the Menu ☰, choose Invoices.
2. Select an existing transaction, or create a new one.
3. Add any necessary information.
4. Click Done.
5. Have your customer sign with their finger.
6. Hit Done.

For iOS, perform the indicated steps: 

1. From the Menu ☰, select Invoices.
2. Choose an existing transaction, or we can create a new one.
3. Add any necessary information.
4. Hit Save.
5. Select Get Signature.
6. Have your customer sign with their finger.
7. Click Done. 

Here's an article for more details: Capture signatures using the QuickBooks Online app. 

Also, I've added this guide about how you can personalize your sales forms in QBO: Customize invoices, estimates, and sales receipts in QuickBooks Online.  

For any inquiries regarding invoice management in QBO, please comment below. Stay safe!