I want to add the type of unit for a property (centimeters) like width [1] using Quickstatements [2].
In the UI it is pretty easy and visible like:
Adding a statement with additional qualifier[3] seems not to work, as I don't know the correct Wikidata property.
I tried to use: type of unit for this property [P2876] like:
Q123384875|P2049|93|P2876|Q4407906
Well:
In your specific case, you can use:
Q123384875|P2049|93U174728
Note that the centimeter (as unit of length) is Q174728, not Q4407906.
The data type that you want to specify is a quantity. Instead, the syntax
Q123384875|P2049|93|P2876|Q4407906
is used for adding a qualifier to the statement (see Help:QuickStatements § Add statement with qualifiers).