yes - it's possible, but depends if someexternalapp.com supports such configuration.
they'll need to add vhost for your mydomain.com.
only thing you need to do is to point your domain mydomain.com to someexternalapp.com
in ideal world you should use CNAME record but this might not work if your domain is literally something.tld, but it will work if your domain is something.somethingelse.tld [ eg www.whatever.com ]. your dns entry would look as follows:
www IN CNAME someexternalapp.com.
[mind the dot at the end of the line ].
alternativly you can point directly to the ip of someexternalapp.com
@ IN A 126.96.36.199
www IN A 188.8.131.52
in this case - make sure someexternalapp will inform you whenever they change their ip.
you'll probably apply those changes via some web gui offered by your domain reseller - concept remains the same, but interfaces might differ.
there is also ugly solution html FRAME on mydomain.com hosted by you that someexternalapp.com but this might look suspicious from your client's perspective.