[Piotr]

You should do this by finding this button by id or class or if this is only one submit button on page by selector $('input[type=submit]');
Im not exactly know how your code should work with this customerId but i think you want to disable button when customerId gonna be empty?

Code:

$('#mytable tr').each(function()
{
  if (!this.rowIndex) return; // skip first row
  var customerId = this.cells[3].innerHTML;
  if(customerId == ''){  // Im understand that innerHTML return text value
     $('#mysubmitbtn').prop('disabled', true);
   }
});