How do you make an entity that spawns with item rightclicked

Make a procedure for when right click on block that looks like this

In the empty box put the item that you want to use, where it says entity go through the list and find your mob

Also feel free to check out my mod Horror Movie Monsters

then just look down and spawn it at your feet... right click the block beneath the one you’re standing on.

With 1.9.1 of Mcreator we can now execute command functions as the console seamlessly at locations and etc,
Simply do this,

Instead of the spawn block in the procedure the other guy suggested,

create that procedure and replace the spawn block with a command block that executes a command at an X, Y Z location, nothing more nothing less.

Then input this command into the command slot,

execute @p ~ ~ ~ /summon [input here] ~ ~ ~ 

replace the [input here]
with the entity ID name of the mob - entity..

you can find this id via going into a test build of your mod via the play button and
pressing T with commands turned on, on that world

Then type /summon (the id of your mod) a : right after it with no space and then press tab, keep pressing tab until you find an ID that looks like its of the mob you want to spawn,

then simply press the end key, hold shift and then keep tapping the left arrow key until you select the entire entity name, including your mod ID,

then press CTRL + C
and CTRL + V over the [input here] area.. it should look something like

execute @p ~ ~ ~ /summon mymod:omegazombie ~ ~ ~

you can find your mod's id via going into your workspace settings under the workspace tab btw.
 

that should work..

Can I ask where is that block "item in entity's main hand"?

Yea, i want to know too

I managed to make it work on 1.18.2 with this procedure, wich i activate with the :"when right clicked on block" trigger:

Spawn at x: "x"  y: "y+1"  z: "z" entity:"[entity id here]"  

(excuse me but my internet is currently so bad i cant send images)