How to merge an HFS+ partition into my APFS partition

Question

I want to merge an empty HFS+ partition with my Mac's APFS partition. My Disk Utility interface looks like this :

How can I do this ?

Answer 1

1. Choose the internal drive (Apple HDD...)
2. Hit "Partition" in the top row
3. Choose the freeMacPartition slice of the pie
4. Hit the --button and then the Apply-button

The freeMacPartition has to be located downstream of (or "after") the APFS container in the partition table or merging won't work.