I have an agency table and I am using a lookup from the Agency table to add multiple agencies to my main record as a multi-select dropdown. I have a model-driven app with this feature but when I try to replicate the same feature in power pages for my public-facing site, the subgrid does not have a "add existing" button

I can use the "add existing" in model-driven app as shown here
But,

The same subgrid does not have that option. How can I fix this?
Thanks