How to use @{-1} with push?

I have an alias that I'm working on to do a push and delete of a topic branch:

    # Push HEAD, delete branch local & remote
    #
    # $1 = remote name
    # $2 = branch name
    pushdel = "!f() { : git push ; git push \"$1\" HEAD \":$2\" && git
branch -d \"$2\" ; }; f"

I use it after I merge a topic branch, like so:

$ git pushdel origin my-topic-branch

What would be nice is if I could do one of the two:

$ git pushdel origin - $ git pushdel origin @{-1}

Both should refer to the last branch, but I know that these can't be used verbatim in a push command because it doesn't read it as a branch name normally like `git checkout` would. I'm not sure why, as I do not have knowledge of the underlying mechanics of these commands.

Is there a way I can use these "relative" branch shorthands in my push command? If not directly, perhaps there is a way I can update my alias to do an intermediate translation?

Thanks in advance.