Is your feature request related to a problem? Please describe.
I'm always frustrated when I see that you can't use itemjoin commands in the issued measure.

1. you can't use an itemjoin command in an execute command.
   Example:
   execute as @A run itemjoin get ....
2. you can't use @A, @p, @s, @r, @e in an itemjoin command.
   Example:
   itemjoin get item_1 @A[distance=..2,team=Team1]

Describe the solution you'd like
I would like it if this would work and you could also use this in command blocks.

Describe alternatives you've considered
I found out that you can give itemjoin items with /give, adding the tags that would normally be at the item of itemjoin. But this automatically gives 2 instead of 1, so I always cleared one, but this also resulted in ghost items.

Additional context

Hi! Sorry it has taken so long but this feature is now available in the latest snapshot!
It has been added exactly as you detailed, you can now use entity selectors in any section where a username would be present, example; /itemjoin get item_1 @a[team=afk]

The only minor thing is there will not be a "complete" tab-completer to assist. It will only give the default suggestions; @a, @e, @p, @r, @s. This is simply a limitation of the spigot API as it is not currently feasible to dynamically show the available values.

To clarify though this doesn't stop you from using them, as shown @a[team=afk] works despite only @a showing in the auto-complete.

Additionally, this functionality has been tested to work properly in command blocks.

Link; https://ci.craftationgaming.com/downloads
Remember to remove your old ItemJoin.jar file

Let me know!~
Thanks for submitting this feature request and for being so patient!

EDIT:
The official release is now available.
Link; https://www.spigotmc.org/resources/itemjoin.12661/download?version=557378