You can create an assignment by using drag and drop.
Procedure
Open the relevant model.
Click Navigation in the bar panel if the Navigation bar is not activated yet.
In the Navigation bar, activate the Explorer tree tab.
In the Explorer tree, open the group in which the model that you want to assign is located.
Click the name of the model and in the modeling area drag it to the object to which you want to assign it.
Release the mouse button. You are asked whether you want to assign the model or open it.
Click Create assignment.
The model will be assigned.