Just going to try to summarize my personal experience on this topic over the past week.
my wife is in a similar boat. Indian passport had only Given Name. Acquired Canadian citizenship. Candian passport has only Last Name (full name in last name).
I called BLS and was told their instruction for regular/paper visas and OCI is to put “XXX” in the Given Name when applying for such cases.
However, I read multiple accounts of people requiring affidavits to support this “XXX” on their visa/OCI. In addition, BLS do not own the process for e-visas.
Called e-visa helpline and they told me to put the entire name in Given Name and leave Surname blank (just like it was on the Indian passport).
She got the visa but we haven’t travelled to India yet. Travelling next month and will share an update on how it goes.