Custom action button at productdetails

[Douglas Ruiz]

Hello, how create custom action button?
i mean like "Cancel" button but run my own code?

Link example: clientarea.php?action=change&id=40
where change will be my function, i need it to run at same tpl productdetails

like here for example, i need display a message when click at my button.

{if $modulecustombuttonresult}
    {if $modulecustombuttonresult == "success"}
        {include file="$template/includes/alert.tpl" type="success" msg=$LANG.moduleactionsuccess textcenter=true idname="alertModuleCustomButtonSuccess"}
    {else}
        {include file="$template/includes/alert.tpl" type="error" msg=$LANG.moduleactionfailed|cat:' ':$modulecustombuttonresult textcenter=true idname="alertModuleCustomButtonFailed"}
    {/if}
{/if}

i created the button
<a href="clientarea.php?action=change&amp;id={$id}" class="btn btn-block btn-info">Trocar Máquina</a>

but i don't know the next step.