That is usually done with the Send Later add-on:

https://addons.thunderbird.net/en-US/thunderbird/addon/send-later-3/

https://support.mozilla.org/en-US/kb/installing-addon-thunderbird

Thanks. I found this after I posted but it's not compatible with the more recent version of thunderbird.

HeatherUpFit said

Thanks. I found this after I posted but it's not compatible with the more recent version of thunderbird.

It is compatible with the release version that being 60.x. So what version are you using? Perhaps you need to use the release version.

version 65 beta 2  :(

I really do not know why we have folk using beta in support forums complaining that add-ons do't work for them. I know add-on authors that are still supporting their add-on make every effort to make their add-ons work in the release version, but that does not automatically mean the beta or daily build will work.

At this time almost no add-ons work with the beta builds and none that I am aware of for the daily builds at all. By the time we have a new Thunderbird release version it may be that the entire API for add-ons will change from what is in the current release, much of this is still in a state of flux but the hybrid that is available in beta is unlikely to ever see it to a release version. So get used to doing it without add-ons if you are staying with the beta builds.

HeatherUpFit said

version 65 beta 2  :(

Here is the reply from the add-on author concerning TB 60+:

https://support.mozilla.org/en-US/questions/1239860#answer-1185907