XF 2.1 How can i call a function with a method post

[luispulido9]

I have this code in mi html page node

<xf:form action="{{ link('paginas/tienda') }}" method="post">
<xf:button icon="preview" name="buy">Buy</xf:button>
</xf:form>

how can i call a php function when i click that button?