The government have said that most people who are self-employed will be able to claim a grant though the (COVID-19) Self-employment Income Support Scheme.
You cannot apply for this scheme yet.
HMRC will contact you if you are eligible for the scheme and invite you to apply online.
HMRC have said you do not need to contact them and doing so will only delay the introduction of the scheme.
I can’t see HMRC contacting you before the end of April because the deadline for submitting tax returns has been extended to April 23.
My feeling is that HMRC will contact you towards the end of May (that’s based on my experience of dealing with HMRC over many years).
Hopefully, it will be sooner rather than later.