foundry/Data/systems/pf2e/templates/items/equipment-details.hbs

{{#if isApex}}
    <div class="form-group">
        <label>{{localize "PF2E.Actor.Character.Attribute.Apex"}}</label>
        <div class="details-container-flex-row">
            <select name="system.apex.attribute">
                {{#select item.system.apex.attribute}}
                    {{#each attributes as |label slug|}}
                        <option value="{{slug}}">{{localize label}}</option>
                    {{/each}}
                {{/select}}
            </select>
            {{#if mandatoryTakeOnce}}<input type="hidden" name="system.maxTakable" value="1" data-dtype="Number" />{{/if}}
        </div>
    </div>
{{/if}}