Go to Model-->Model Properties-->Environments tab-->General and set the curreent workspace environment and the scratch workspace environment there.
Hit OK
Now use current workspace as %workspace% and scratch workspace as %scratchworkspace% in your input and output path respectively.
0r 2)
Right click anywhere in your model and from the side menu click Create Variable. Create a variable of type workspace. You can rename the variable to anything say: Input Workspace.
Provide the variable a workspace value and use it in other tools as %Input Workspace%.
**Its always recommended that you set the environments at the ArcMap/application level if all your models are using the same workspace.
---------------------------------------- Iterate though a list of workspaces or the feature classes and tables inside the workspaces? What do you want to do by iterating over workspaces?