Steps to create Property Promotion field in Infopath form and map it to library :-
1. Open InfoPath form in design mode.
2. Create Property Promotion field from Form Option
3. Save InfoPath form as source.
4. Note down XPath of property promoted field
5. Open XSF file notedown the Guid or Change the Guid based on field GUID
6. Create Field with node value of as of XPath
7. Deploy Field in Site
8. Deploy Infopath Form using Custom Content Type and bind field to the Content Type
Now go to the Form Library all the Fileds will be available. 🙂