You need to "export" your MyHeritage data to a Gedcom file. If I remember correctly, "Export" is an option under the File tab in the MyHeritage FamilyTreeBuilder software. See https://www.wikitree.com/wiki/Help:Exporting_a_GEDCOM for information about some of the settings to use when you create your export file. That file you export can then be uploaded to WikiTree for GEDCOMpare, where you compare your tree with data currently in WikiTree, and (after processing) you can selectively import your data for people who aren't already in WikiTree. Note that there's an upper limit of 5000 profiles for a GEDCOM to be uploaded, and for practical reasons, most of us will urge you to start with a much smaller file than that. You can limit the size of a GEDCOM in several different ways -- for example, you could export all descendants and descendants' spouses and siblings for a particular 6G grandparent.
