LeadSquared's API host depends on your account's region (e.g. api-in21.leadsquared.com, api-us11.leadsquared.com). Keys go in the query string.
Find your access key, secret key, and API host under Settings → API and Webhooks. In the examples they're {{LSQ_ACCESS_KEY}}, {{LSQ_SECRET_KEY}}, and {{LSQ_HOST}}.
Full reference: https://apidocs.leadsquared.com/
curl "https://{{LSQ_HOST}}/v2/LeadManagement.svc/Leads.GetByEmailaddress?accessKey={{LSQ_ACCESS_KEY}}&secretKey={{LSQ_SECRET_KEY}}&emailaddress=jenny%40example.com"
Fields are a list of Attribute/Value pairs.
curl -X POST "https://{{LSQ_HOST}}/v2/LeadManagement.svc/Lead.Create?accessKey={{LSQ_ACCESS_KEY}}&secretKey={{LSQ_SECRET_KEY}}" \
-H "Content-Type: application/json" \
-d '[{"Attribute": "EmailAddress", "Value": "[email protected]"}, {"Attribute": "FirstName", "Value": "Jenny"}]'