How do I create a segment to target specific users by their UserID?

You do not need a Tag workaround for this. Pushwoosh has a built-in User ID filter in the segment builder.

Build a segment by User ID (recommended)

1. Go to Audience → Segments.
2. Click Create Segment, then Build Segment.
3. In Add filter by, click User ID.
4. Add the User IDs that should be included in the segment. Use User IDs that already exist in your Pushwoosh project.
5. Optionally combine the User ID filter with Tag, Event, Segment or Geo filters, and enable Include all user devices to target every device belonging to a matching User ID.
6. Click Save.

If you need to upload a large list of User IDs or Hardware IDs from a file, use Import a segment instead of typing them in.

If you really want the UserID stored as a Tag

You can additionally write the UserID into a Tag (for example, for reporting), using Update tags via CSV. Note the actual requirements:

• The Tag must already exist in your account — CSV import does not create new Tags.
• The first column must be named HWID and contain the hardware IDs of the devices. Every other column must be named exactly as a Tag in your account; columns that do not match a Tag name are skipped.
• Save the file as UTF-8 with comma delimiters.
• In the Control Panel go to Import CSV → Update Tag values, upload the file, select the columns to import and press Start import.

HWID,user_identifier
XXXX-XXXX-XXXX-XXXX-XXXXXXXXXXXX,user-id-123
YYYY-YYYY-YYYY-YYYY-YYYYYYYYYYYY,user-id-456

For details see the documentation on building segments by User ID and updating Tags via CSV.