Some features allow Equations and linking directly in the Property Manager. (See 2018 SOLIDWORKS Help - Direct Input of Equations in PropertyManagers for the list.) It's as simple as typing the Equal sign in the dimension box to get the drop-down where you can make your selection. See below from a Linear Component Pattern in an Assembly.
This hasn't always been the case, and still isn't available for all features that use a dimension, but it can still be done. If you can't do it directly in the Property Manager go ahead and create the feature, using a value that's close to what you need, and click Okay to close the feature. Now there are two options for linking the dimension.
1. Single-click on the feature in the tree to show the dimension in the graphics area (or double-click if you don't have Instant 3d turned on).
Double-click on it to bring up the standard dimension dialog box, enter the Equal sign, and you'll get the same drop-down shown above. You'll need to do a manual rebuild for the change to take effect.
..or option 2: Open your Equations dialog box and go to the "Dimensions" tab. All dimensions in the model will be listed. Remove the value in the "Value / Equation" column and enter the Equal sign. That will give the same options for linking. (Naming dimensions will be a big help if you want to use this option. That will make it much easier to find the one you need.)
By the way, I believe in giving credit where it's due. I learned about this second option from Frederick Law at Global variables in "Thicken" feature?.
Just testing to see what the comment on the rating shows up like. It IS well written and all that.