To enable marketing integration, enable this setting in General settings:

This should show a Marketing Integeration section in General settings. 
Enter API key there, and it should start syncing the customers.
The system will sync all standard attributes from point of sale to GoHighLevel.

If you want to attach custom tags from point of sale, you can enter a comma-separarated list of tags into CUSTOM TAGS MAPPING field.
You can also map specific attribute values from POS to a custom tag in GoHighLevel, example:

dob_month|1,01 January bdays
dob_month|2,02 Febuary bdays
dob_month|3,03 march bdays
dob_month|4,04 april bdays
dob_month|5,05 may bdays
dob_month|6,06 june bdays
dob_month|7,07 july bdays
dob_month|8,08 august bdays
dob_month|9,09 september bdays
dob_month|10,10 october bdays
dob_month|11,11 november bdays
dob_month|12,12 december bdays

The integration then will take dob_month attribute, and for attribute value 1, it will map it to a tag 01 January bdays.

CUSTOM FIELDS MAPPING can be used to map fields to GoHighLevel fields.
You can map specific values in point of sale to values in GoHighLevel, the format in CUSTOM FIELDS MAPPING should be:

Then the integration will try to take attribute with code <pos_attribute_code> on the customer and put it into <gohighlevel_attribute>