abbrev-prefix-mark is an interactive compiled Lisp function in `abbrev.el'.

It is bound to M-'.

(abbrev-prefix-mark &optional ARG)

Mark current point as the beginning of an abbrev.
Abbrev to be expanded starts here rather than at beginning of word.
This way, you can expand an abbrev with a prefix: insert the prefix,
use this command, then insert the abbrev. This command inserts a
temporary hyphen after the prefix (until the intended abbrev
expansion occurs).
If the prefix is itself an abbrev, this command expands it, unless
ARG is non-nil. Interactively, ARG is the prefix argument.