You cannot call API test from Function library. For a work around most customers create a GUI Test that calls the API test using the RunAPITest Statement.

Please review this statement in the UFT user guide because it has the following statement.

IMPORTANT

You add this statement using the Call to API Test/Action dialog box. You cannot add this statement manually in the Editor. For more details about the Call to API Test/Action dialog box, see the HP Unified Functional Testing User Guide.

The Service Test or API test is loaded only when the step runs (and not when the test opens). This can affect test maintenance because no warning is displayed if the called Service Test or API test is moved or deleted.