1. Go to Financials > Expense Types and click on the expense type that you wish to link.

2.a If you wish the expense type to be posted to Xero as a Bank Transaction, ensure that the Expense Type is set up as an Account Transaction and is linked to a Bank Account.

3. Alternatively, you may choose to post the expense to Xero  as a Supplier Invoice:

In this instance you would link the Expense Type to a Xero Expense Account and choose Supplier Invoice / Bill as the posting to Xero option.