As Kevin stated, the HTML is populated when the form is initially loaded, so that is before the value was populated.  Here's some Javascript (for the Classic Designer, since that is how your post was tagged) that checks for changes to the field and updates the link.  To use this code, add the Class name of taxMap to your TaxMap field, and the Class name of taxMapLink to your HTML element with the link: 

$(document).ready(function() {
  
  //When the field with the taxMap class is updated,
  //update the HTML link using that value for the secondary form.
  $('.taxMap input').change(function() {
    let taxMapValue = $(this).val();
    let taxMapHTML = '<p><a href="URL_GOES_HERE?Tax_Map_ID__=' + taxMapValue + '" title="TITLE_GOES_HERE" target="_blank"><b><font size="3">Link to Apartment Survey Form</font></b></a><br></p>';
    $('.taxMapLink').html(taxMapHTML);
  });
  
});