The tags for those fields in links.db are not used in add.html or modify.html. The reason is, you have not passed any record values to the load_template routine at that point. Instead, you use normal <input> tags in the add.html (for example) template with the name value set to the field's name. For the AltCategories, you need to create a <select> list in sub site_html_add_form of site_html_templates.pl, and then pass it to the template via load_template. Look in that subroutine to see how it is similarly done for the Category field.

I hope this helps.