As far as I know this isn't possible. According to the documentation these are your options:

You can place the automatically generated number anywhere else in the name by using a token. When you type the pool name, type n surrounded by curly brackets to designate the token.

For example: amber-{n}-desktop

When a machine is created, Horizon 7 replaces {n} with a unique number.

You can generate a fixed-length token by typing {n:fixed=number of digits}.

Horizon 7 replaces the token with numbers containing the specified number of digits.

For example, if you type amber-{n:fixed=3}, Horizon 7 replaces {n:fixed=3} with a three-digit number and creates these machine names: amber-001, amber-002, amber-003, and so on.

Length of the Naming Pattern When You Use a Fixed-Length Token

Names that contain fixed-length tokens have a 15-character limit, including your naming pattern and the number of digits in the token.