Fairly simple once you know how.

Servers must have the following commands set to provide autodownloadable addonpak content:

seta net_serverDlTable "/"
seta net_serverDlBaseURL "http://"
seta net_serverDownload "2"

For example:

seta net_serverDlTable "q4base/map-l4dm1x.pk4"
seta net_serverDlBaseURL "http://www.quakecommunity.com"
seta net_serverDownload "2"

You then have to put the map pak on a webserver to host the autodownload, so I have to put the map available for download at http://www.quakecommunity.com/q4base/map-l4dm1x.pk4

Now clients with net_clientdownload "2" set, will autodownload the map from the webserver. It is all automated. While the file is supplied from a http daemon and not from the game servers q4base, you still remain in-game while its downloading, get to view the file progress, and you will connect and the map will load automatically once the download is complete.

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See this existing thread on how to make a custom map a proper AddonPak: https://www.esreality.com/?a=post&id=1033582