How to enter custom format for both current date and time in cell?

I know we can enter a date and time into a cell separately. But I'd like to create a custom format that allows me to enter both the date and time stamp concurrently in one cell. However when I select the custom format and create such a format it enters the date correctly but not the time. The time defaults to 12:00 PM even though I'm asking for the custom format to time stamp the current time into that cell using the custom format.

What am I doing wrong? Is there a way to get a custom formatted cell to input both time and date stamp correctly simultaneously in the same cell or do I have to create a separate cell for the time stamp? Thanks in advance.