This link to Readdle’s knowledge base for the Spark’s url scheme is no longer valid.
This link provides a bit more detail:
readdle-spark://
Start draft
readdle-spark://compose
Identifiers to tack onto the start draft scheme
[add “?” after “compose” for first one]
[then add “&” for each additional one]
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Add recipient = recipient=TheirEmailAddress
Add email subject = subject=YourSubjectText
Add email body text = body=YourBodyText
Add cc recipient = cc=TheirEmailAddress
Add bcc recipient = bcc=TheirEmailAddress
Examples of identifier combinations
Start draft with recipient
readdle-spark://compose?recipient=TheirEmailAddress
Start draft with recipient, subject
readdle-spark://compose?recipient=TheirEmailAddress&subject=YourSubjectText
Start draft with recipient, subject, body
readdle-spark://compose?recipient=TheirEmailAddress&subject=YourSubjectText&body=YourBodyText
Start draft with recipient, cc, bbc, subject, body
readdle-spark://compose?recipient=TheirEmailAddress&cc=TheirEmailAddress&bcc=TheirEmailAddress&subject=YourSubjectText&body=YourBodyText
UPDATE April 21, 2022 2:30 AM
Deleted incorrect usage of the term “x-callback-url” since this scheme does NOT support callback. Sorry for the mistake.