Adding php do_shortcode to array in WordPress

I’ve got this code here:

$defaults = array(
 'id' => 'private_message-'.$user_id,
 'component' => 'messages',
 'must_be_logged_in' => true,
 'block_self' => true,
 'wrapper_id' => 'send-private-message-'.$user_id,
 'wrapper_class' =>'send-private-message',
 'link_href' => hibuddy_get_send_private_message_url(),
 'link_title' => __( 'Ask the model a question about this item!', 'buddypress' ),
 'link_text' => __( 'Send Seller a Private Message', 'buddypress' ),
 'link_class' => 'send-message',
 );

And I’m trying to change the line that says:

 'link_text' => __( 'Send Seller a Private Message', 'buddypress' ),

…to:

'link_text' => __( 'echo do_shortcode('[button]'.Send Seller a Private Message.'[/button]')', 'buddypress' ),

…but it doesnt like it. In a nutshell, I’m trying to change the “Send Seller a Private Message” text to output [button]Send Seller a Private Message[/button] which requires the WordPress do_shortcode.

Thank you!