Check the searchhelp definition via transaction SE11. All "import" parameters can be used (checkbox in column "Imp"):

InputField (12,2) "Material number" (12,20) size=10 _
   name="matnr" searchHelp="MAT1" _
   shSelName1="WERKS" shSelvalue1="0001"

This mechanism does not work if the parameters are not marked as "import" parameters. For example in searchhelp VMVAA:

GuiXTCopy

InputField (12,2) "Order number" (12,20) _ 
   size=10 name="aufnr" searchHelp="VMVAA" _
   shSelName1="TRVOG" shSelvalue1="0"

"Transaction group" is empty, and so sales orders, inquiries, quotations, contracts,... will be selected unless the user specifies "0" in this field for sales order selection.
In the case of searchhelp "VNVAA" there is an easy workaround, since the user parameter id VTV is specifed for the parameter TRVOG (column "Default value" in the searchhelp definition). Using the notation "$VTV" as parameter name you can address this user parameter:

GuiXTCopy

InputField (12,2) "Order number" (12,20) _ 
		size=10 name="aufnr" searchHelp="VMVAA" _
   		shSelName1="$VTV" shSelvalue1="0"

This trick does not work for the other parameters. To handle these, copy the SAP searchhelp and change the "import flag":

Now all parameters can be addressed:

GuiXTCopy

InputField (12,2) "Order number" (12,20) _ 
	size=10 name="aufnr" _
   searchHelp="/GUIXT/VMVAA" _
   shSelName1="VKORG" shSelvalue1="1000" _
   shSelName2="VTWEG" shSelvalue2="10" _
   shSelName3="SPART" shSelvalue3="00" _
   shSelName4="TRVOG" shSelValue4="0"

Components
InputAssistant