Two possible fixes here, we can change Tor Browser to use the existing 'I usea proxy to connect to the Internet' tor-launcher string, or we can updated tor-launcher to add the new 'Use a proxy' string.
I think it's worth reusing the string people might recognize from dealing with the Tor Launcher dialog. Thus, I picked the tor-browser option: commit 94f2e5e7e73c3b5b52055eadd0e2dcc864d598c4 on tor-browser-68.2.0esr-9.0-1 and commit cbf86255ae7cfe82b555931d9db3b9e528036c65 on tor-browser-68.2.0esr-9.5-1.
Trac: Resolution: N/Ato fixed Status: needs_review to closed