Hi,
If it's custom HTML, you have the same kind of triggers in HikaMarket than in HikaShop, if you want to add "blocks", like HikaSerial or HikaAuction are adding (onMarketProductBlocksDisplay).
Otherwise, if it's just some JavaScript, you can use the regular Joomla function to add it into the header.
Regards,
Jerome - Obsidev.com
HikaMarket & HikaSerial developer / HikaShop core dev team.
Also helping the HikaShop support team when having some time or couldn't sleep.
By the way, do not send me private message, use the "contact us" form instead.