1. Create the custominput component under /apps.my-site. Make sure you follow an implementation of how an existing data type has been implemented for the content fragment editor under /libs/settings/dam/cfm/models/formbuilderconfig/datatypes.
2. overlay /libs/settings/dam/cfm/models/formbuilderconfig/datatypes with /apps/settings/dam/cfm/models/formbuilderconfig/datatypes.
3. Use the Sling Resource Merger mechanism to add your custominput component under the datatypes node. Add the new node, /apps/settings/dam/cfm/models/formbuilderconfig/datatypes/custominput with configurations like (making sure that you take an existing content fragment data type component as inspiration and as an example for your own configuration, so you understand the structure of how to component is setup; also lets you understand the requirements to how you can create your own datatype component.