Hi Queralt,
there are two ways to create top-level parameters: an 'easy', semi-automatic one, and a more flexible, manual one.
In your case, i.e. same parameter across multiple blocks, you can multi-select the blocks (either in the Layout or in the Model Explorer pane) and then drag-drop the parameter to the Top-level Quantities sheet / Parameters tab: a corresponding top-level parameter, with the same data type, units etc. of the original parameters will be automatically generated, in particular the Links properties is automatically set to the individual sub-model parameters.
The other way requires that you manually type in Name, Data Type, etc. for the top-level parameter and especially select one by one the Links to the underlying sub-model parameters.