specify packages in the YAML header

aastex62/aastex62_template.tex

@@ -12,6 +12,14 @@ $if(bibstyle)$
 \bibliographystyle{$bibstyle$}
 $endif$
 
+$if(texpackages)$
+\usepackage{
+$for(texpackages)$
+$texpackages$,
+$endfor$
+}
+$endif$
+
 $if(preambleinput)$
 $for(preambleinput)$
 \input{$preambleinput$}

aastex62/demo.md

@@ -1,6 +1,7 @@
 ---
 aastexopts: [singlecolumn]
 preambleinput: ["macros.tex"]
+texpackages: [xspace, amsmath]
 bibstyle: aasjournal
 received: "January 1, 2018"
 #revised: "January 7, 2018"
@@ -81,6 +82,7 @@ The YAML header given below is that used for the preparation of this document:
 ---
 aastexopts: [singlecolumn]
 preambleinput: ["macros.tex"]
+texpackages: [xspace, amsmath]
 bibstyle: aasjournal
 received: "January 1, 2018"
 #revised: "January 7, 2018"