To make an input in a cell mandatory without leaving it blank, you can use the Data Validation feature in Microsoft Excel. Here are the steps:

1. Select the cell or range of cells where you want to apply the mandatory input.
2. Go to the Data tab in the ribbon and click on the Data Validation button.
3. In the Data Validation dialog box, choose "Custom" from the Allow drop-down list.
4. In the Formula box, enter a formula that checks if the cell is not blank. For example, you can use the formula
   Formula:

   =LEN(A1)0 

   if you want to make sure that the input in cell A1 is not empty.
5. Optionally, you can add a custom error message to inform the user about the mandatory input. Click on the Error Alert tab and enter a title and message for the error.
6. Click OK to close the Data Validation dialog box.

Now, if the user tries to leave the cell blank or enter an invalid input, Excel will display the error message and prevent them from saving the worksheet until the input is corrected.