Create a system user in AEM using runmode config

Add/create a system user through run mode in AEM using ACS commons. Create a run mode config file in your project.
/apps/aemquickstart/config/com.adobe.acs.commons.users.impl.EnsureServiceUser-customServiceUser.config

Add the below snippet in the config file

principalName="custom-service-user"
type="add"
ensure-immediately="{Boolean}true"
aces="[type=allow;privileges=jcr:read\,jcr:modifyproperties;path=/content/we-retail;rep:glob=/jcr:content/*,type=allow;privileges=jcr:read;path=/content/aemquickstart;rep:glob=/jcr:content/*]

The above snippet will add read and modify properties privilege to /content/we-retail path and read permission to /content/aemquickstart.

Click here to check how to create a system user using system explorer

Refer ACS Commons documentation