Sure thing!
First off, you have to decide what type of data you want to store and create a proper attribute type for this kind of information. You can choose from text, email, list with options, status, URL or IP and name the attribute whatever you like. After creating the attribute, it is not directly visible on CI detail page as only attributes with value are being displayed. When you click the edit option, it should be now accessible from that screen and can be filled with a value.
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Note: Remember to use custom data with proper Missing Objects import configuration policy (Ignore or Update) so the data won’t get deleted after it is no longer avalable through the Dynatrace API - unless you want it to. With this setup also after the entity is no longer monitored, data stays in your CMDB as long as it does not get manually deleted by a user.