expl3 test if a string starts with a given prefix

Using expl3, I'd like to test if a given "string" (argument) starts with
a given prefix, namely "http://". For a prefix of a given length, 7 in
my case, I guess the idea is to extract the first 7 tokens using
str_range and then compare them with \str_if_eq? But I cannot get the
details right. Here's my best attempt (with help from various bots). Is
it at all close? 

\documentclass{article}
\begin{document}
\ExplSyntaxOn
\cs_new:Npn \astr_if_starts_with:nnTF #1#2
  {
    % #1 = prefix to test
    % #2 = full string
    \str_if_eq:nnTF
      { \str_range:nnn {#2} {1} { \str_count:n {#1} } }
      { #1 }
  }
\str_if_starts_with:nnTF {abc} {abcdef}
  { \message{prefix} }
  { \message{notprefix} }
\end{document}

This feels like an easy expl3 programming question, but evidently I've
failed to grok interface3.pdf.

FWIW, the real context is this, from tugboat.dtx (with thanks to
Ulrike):

\def\tbsurl@#1 
 {
   \str_set:Nn\l_tmpa_str{#1}
   \str_if_in:NnTF \l_tmpa_str {http://}
      ... % an http:// url

where I need to replace \str_if_in with some other function that checks
if http:// is only the *beginning* of \l_tmpa_str, not anywhere within
it.  Because archive.org routinely embeds "http://" in their urls,
e.g., https://web.archive.org/web/20090809184749/http://www.eco-log.de/

Thanks,
Karl