When purchasing hashing power, you must specify your stratum server setting. Stratum is a protocol that allows mining pools to communicate with miners. Mining pools will specify their stratum server settings in their help pages, so please consult them to figure out what to put in the forms.
They may specify their stratum configuration as an URL or address, for example "stratum+tcp://stratum.slushpool.com:3333" or "stratum.slushpool.com:3333". In this case stratum.slushpool.com is the Stratum Host, and 3333 is the Stratum Port. Stratum Username is generally the username you registered at the mining pool, followed by a worker name such as "username.workername" but this is different from pool to pool so you should consult their documentations. Stratum Password is generally not required, but again this is also different from pool to pool so consult their documentations.