How can I change the data in the Expectation field for a custom expectation?

How can I change the data in the Expectation field for a custom expectation?
“success”: actual_table_row_count == expected_table_row_count
changes the status icon (Status)
“result”: {
“observed_value”: {“total_rows”: actual_table_row_count},
changes only the last column (Observed Value)
what about the Expectation field?