We need the WorkshopID for this. You can find it in the link.
As an example, we will use the mod CBA (Community Based Addons).
The ID for CBA is: 450814997.
If your Arma 3 configuration is in Basic Mode, the WorkshopIDs need to be entered under
Mod Settings in the field
Mods (Automatic Installation via Steam Workshop).
If you have multiple mods, separate the WorkshopIDs with semicolons.
It would look like this:
In Advanced Mode, you will find the field
Mods (Automatic Installation via Steam Workshop) under the
The mods should be entered here following the same rules as in Basic Mode.