I don't believe you can. I only have the free version, maybe the pro version is different.
Brother, I am using professional version (29 days trial). But there is nothing like constraints. If u have no idea about constraints than no problem.
As you are creating entities, you can use snaps to set them initially, but these are not dynamic.
If you use one and move one of the entities, the snap you created doesn't update.
I recommend that you do not use "Nearest".
While you are in the middle of creating an entitiy, you can type the first 3-4 letters of the names above and that snap will be forced.
For example, if you're creating a line and want its endpoint to be in the middle of another line, type "mid" before you click the endpoint and mouse near the middle of the other line,
But you can see when I drag one line, the line snapped to its middle doesn't follow.
To my knowledge, DraftSight doesn't operate using constraints.