What version of AutoCad you are using? I hope it already have the "Additional format" button.
But in the position field, the thousand separator is grayed out. I only saw it enabled on area properties. Nevertheless, you can still use it, just need an extra work.
In the Field Editor, the field expression will look like this.
The additional format is defined by what is inside the quotation marks next to "\f". But AutoCAD prevents us from editing the field expression so we need a workaround. Copy the field expression to clipboard. Paste to Notepad then add "%th32" in the format. That is, using a space (ascii character 32) as thousand separator. The expression will now look like this.
%<\AcObjProp Object(%<\_ObjId 8796084288624>%).InsertionPoint \f "%lu2%pt1%ds44">%
Again copy. Then switch to AutoCad, edit the text (not in the field editor) and just paste it over.
%<\AcObjProp Object(%<\_ObjId 8796084288624>%).InsertionPoint \f "%lu2%pt1%ds44%th32">%