To send lumens (XLM) you will either need an exchange account or a wallet with Stellar lumens. (See “Where can I buy lumens?” help center article if you need help with getting your first Stellar lumens)
To activate your Hot Wallet you will need to send at least 1.5 XLM to your Hot Wallet’s Public Key. The Public Key is shown on the Hot Wallet onboarding page and can be copied from there. Once copied, go through the withdraw/send flow in your wallet or exchange and input the Hot Wallet Public Key as the recipient.