The option to add a custom tax can be found in the Gear icon when using a mobile version, @accounts615.

Please know that these steps will apply to both iOS and Android phones.

Here's how:

1. Open QuickBooks Online and click Menu.
2. Select the Gear icon in the upper-left corner and tap Tax Rates.
3. Click the + button in the lower-right corner and choose the type of sales tax to set up.

If the option isn't visible, you can troubleshoot your mobile app. Cached data can prevent certain features from appearing. Here's how:

For Android:

1. Go to Menu ☰, then tap More Options ⋮
2. Tap Settings, then Refresh Data.
3. Tap YES to confirm.

For iOS:

1. Go to Settings, then tap General.
2. Tap iPhone Storage, then tap the QuickBooks Online app.
3. Tap Offload App.

Alternatively, uninstall and reinstall the mobile app for a complete reset if needed.

We're always here to support you if you need further assistance with QBO. I'll keep this thread open for your future reference.