Why does metamask default to BNB when a custom purchase tokens has been defined?

I’m currently creating a presale dapp for fun on the BSC testnet, and all is going well. Aside from the fact that when I initiate the “buyButton”, metamask seems to default to purchasing with BNB. I’ve attempted to set a custom token as the purchase currency to buy the presale token with, and this is defined in the presale contract itself. I’m not available to send my scripts at the moment but I was wondering if there’s any general guidance or similar experience that you could share, and what could be potentially causing this?

thanks!

tried different variations of my contract and my JS code, but I can only seem to get the BNB default or no purchase currency at all (metamask still opens, but no currency shows)