If you have a campaign on the platform and you want to pay for your treatment, but the hospital tells you that you have to pay on the spot, then contact your health coordinator or your translator and tell them that you want to pay by proforma invoice because your treatment is funded through a charity campaign.
A document will be issued to you (the so-called proforma invoice) and via it you should request payment from your user profile. How to request payment from your profile - read our guidelines: How can I request payment?