According to DbSet.Find you can pass in the primary keys separated by commas
db.tableModels.Find(PK1, PK2)
The Find method takes an array of objects as an argument. When working with composite primary keys, pass the key values separated by commas and in the same order that they are defined in the model.
However, if you want to just use one value, you will probably have to use
db.tableModels.Where(i => i.partialPK == parameter)
or an equivalent Linq operator