<input type='text' name='promo' />
<input type='submit' name='add_promo' value='ajouter'
onclick="this.form.action += 'add_promo/' + this.form.promo.value;" />
<input type='text' name='promo' />
<input type='submit' name='add_promo' value='ajouter'
onclick="this.form.action += 'add_promo/' + this.form.promo.value;" />