How to pass a variable in a curl command in shell scripting

When using variables in shell, you can only use doubles quotes, not single quotes : the variables inside single quotes are not expanded. Learn the difference between ' and " and `. See http://mywiki.wooledge.org/Quotes and http://wiki.bash-hackers.org/syntax/words

I ran into this problem with passing as well, it was solved by using ' " $1 " '

See connection.uri below

curl -X POST -H "Content-Type: application/json" --data '
  {"name": "mysql-atlas-sink",
   "config": {
     "connector.class":"com.mongodb.kafka.connect.MongoSinkConnector",
     "tasks.max":"1",
     "topics":"mysqlstock.Stocks.StockData",
     "connection.uri":"'"$1"'",
     "database":"Stocks",
     "collection":"StockData",
     "key.converter":"io.confluent.connect.avro.AvroConverter",
     "key.converter.schema.registry.url":"http://schema-registry:8081",
     "value.converter":"io.confluent.connect.avro.AvroConverter",
     "value.converter.schema.registry.url":"http://schema-registry:8081",
     "transforms": "ExtractField",
     "transforms.ExtractField.type":"org.apache.kafka.connect.transforms.ExtractField$Value",
     "transforms.ExtractField.field":"after"
}}' http://localhost:8083/connectors -w "\n"