To mark an expense transaction as an asset, in the web app, click on the expense, then click "Advanced". There will be a drop-down for Expense Type, and you’ll be able to select "Asset" from the options.

There are several Asset categories to choose from. If you don’t see the one you need, you can also create your own custom expense categories. When you create a custom category, it will ask you to select an “Account”; be sure to pick an Asset account.