When a Sharepoint Library has been created with a custom document type, it adds metadata to the documents that are save into that library. If some of those fields are set to 'required' then they become compulsory fields that require a value when loading a document to that library. If your Word form was previously saved into that Sharepoint Library, then it will already contain the required Sharepoint fields and you can edit the contents of those fields prior to loading the document to Sharepoint.

To edit these fields via the Word GUI you can insert a linked Content Control onto the page and edit the value which writes back to the metadata field. The Content control can also be removed/deleted from the document without changing the value stored in the metadata. To get one of these linked CCs, you can use Charles' suggestion of Insert > Quick Parts > Document Properties
Alternatively, if you are showing your Developer Tab, you can go to Developer > XML Mapping and find the relevant xml file and field then right click it to insert the CC as a Plain Text Content Control into your document.

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Andrew Lockton
Chrysalis Design, Melbourne Australia