Ah, understood!
Stated the link with placeholder "[user_id]" as proposed in the backend add-on configuration:
/../my.php?section=123&vendor_user_id=[user_id]
Changed the template code from
[...] src="{$field.values|trim}" [...]
to
[...] src="{$field.values|trim|replace:"[user_id]":$smarty.session.auth.user_id}" [...]
Get of URL querystring "vendor_user_id" on my.php works!
Thank you!
But: this was an modification in the Add-On template. Is there a way not to perform this change directely in the Add-On template (e.g. via Mychanges Add-On)?
Best regards
hummer