Generating input value on 2 previous filled inputs

I am trying to generate a input value based on 2 earlier input values the user filled in. First is a name, the second one is a date. Now I need to concat and format these 2 values and set them for another input.

This is what I already come up with:

1. Event listener for first input: Name
2. Format the Name value
3. Event listener for second input: Date
4. Format the Date value
5. Concat these 2 values
6. Set the concat value in third input

The code I have so far:

    window.onload = function () {
        
        /* event listener for first input: Name */
        document.getElementById('input_7_1').addEventListener('change', formatTitle);
        
        /* function to format the name value properly */
        function formatTitle(){
            document.getElementById('input_7_1').addEventListener("input", function(){
              var title = document.getElementById('input_7_1').value;
              var formatTitle = title.replace(/s+/g, '');
            });
        }
        
        /* event listener for second input: Date */
        document.getElementById('input_7_3').addEventListener('change', formatDate);
        
        /* function to format the date value properly */
        function formatDate(){
            document.getElementById('input_7_3').addEventListener("input", function(){
              var date = document.getElementById('input_7_3').value.replace(///g, '');
              var formatDate = date.slice(0, -4);
            });
        }
        
        /* concat both values in a variable */
        var studentNummer = formatTitle.concat(formatDate);

        /* set value in the third input: Studentnummer */
        function generateInput(){
            document.getElementById('input_7_9').value = studentNummer;
        }
    }

I am almost there for my feeling. What am I missing here?