Search block submission URL can't be customized

[iandunn]

What problem does this address?

The search block always submits to home_url( '/' ), but I'd like to submit to a custom URL.

In my use case, I'm building a header that will be used across all sites in a Multisite network. I want searches to go to a custom page that searches across all of the sites (example).

I want to preserve the ability to search a single site, though, so I'm not overriding WP's normal search behavior.

render_block_core_search() doesn't provide any filters or block attributes that can be used to change the form's action, though.

Related: #35260, #22071

What is your proposed solution?

I thought of two options:

1. Add a block attribute. This would allow customizing the URL on a per-block basis. It seems like more of a dev/FSE feature, though, so it might not need a UI.

   <!-- wp:search {"formActionUrl":"https://wordpress.org/search/do-search.php" ... /-->

2. Add a filter to render_block_core_search()

   add_filter( 'search_block_action_url', function( $url ) {
       return 'https://wordpress.org/search/do-search.php';
   }

I'd lean towards the attribute, but don't feel strongly. They're not mutually exclusive, either.

[iandunn]

WordPress/wporg-mu-plugins#155 has a workaround, which is conceptually similar to solution #2 above.

A block attribute might still be nice, but maybe it's enough of an edge case that the workaround is good enough?