I have prepared two examples for you in this snippet: https://mdbootstrap.com/snippets/standard/grzegorz-bujanski/6202607

The first is the simplest example that forces you to enter numbers or letters. The second one is more complicated and blocks the entry of digits other than 0 or 1 in the first character, or the first digit greater than 5 in the minutes. But both examples cannot guarantee 100% that the user will not enter an incorrect value. For example, in the second example you will still be able to enter e.g. 19:59:59 AM. Input mask does not provide full validation that will enable validating such complex cases