Freemius currently only supports numeric versioning:
major.minor[.build[.revision]]
Assuming you’ll be using a 3 part versioning, for beta versions add .0 as the last componenet of the version.
For example, if you want to release a beta for version 1.2.7, set the beta version to 1.2.7.0. For beta revisions of the same version keep incrementing the 4th component. When the version is out of beta simply ommit the 4th component and set the version to 1.2.7.