You can add the button HTML like this:
add_filter( 'generate_navigation_search_output', 'tu_nav_search_button' );
function tu_nav_search_button() {
printf(
'<form method="get" class="search-form navigation-search" action="%1$s">
<input type="search" class="search-field" value="%2$s" name="s" title="%2$s">
<input type="submit" class="nav-search-submit" value="Search" />
</form>',
esc_url( home_url( '/' ) ),
esc_attr( get_search_query() ),
esc_attr_x( 'Search', 'label', 'generatepress' )
);
}
Then we’ll have to style it with some CSS. Let me know when the above is added, and link me to the page ๐