Calling functions from imported Dll in test complete

What you've done, is correct -

1. You've defined the DLL type. 
2. You've defined the Parameter type
3. You need to declare the Function type e.g. Def_DLL.DefineProc("NavigateTo", vt_string)
4. You've loaded the DLL in memory
5. Now you need to call the Function i.e. Lib.NavigateTo("")

Refer to https://support.smartbear.com/testcomplete/docs/testing-with/advanced/using-external-functions/calling-from-dll/index.html and the examples.