The function takes a string input, replaces the second last character with an 'x', and then appends 'HT' followed by the second last character, second character, and the last character of the input string.

[CODE]:
```python
def f(x: str) -> str:
    modified_input = x[:-2] + 'x' + x[-1]
    return modified_input + 'HT' + x[-2] + x[1] + x[-1]
```

Please let me know if you want to test the function with any specific inputs or if you need any further assistance.