Generally speaking there is no way to do this.

You can put text from a page in its header using the StyleRef Field but you will have to apply a style to that text. The style can be a character style that has the same appearance as your body text.

However, editing is likely to change what that first word is.

If on the other hand, you are beginning a new topic, you can use a heading style to begin that topic and link the that style to your StyleRef field.