Shimano's STX groupset utilizes hyperdrive and Dual SIS technology, so the Shimano chain, CN-HG40 would be an excellent choice for the 7 speed drivetrain you have. Shimano chains in speed classes 7 through 10 speed do not have a master link option. They come with a link pin to join the chain after it's cut to size, however one can instead utilize a master link such as KMC's "missing link" or SRAM's "power link" on the Shimano chain and maintain normal function. Note that the master link of choice must be a match to the speed of the chain. In your case, a 7 speed version of the power link or missing link must be used. If you get a master link for say, a 9 speed chain, you won't be able to get the link to join and close because the width of a 9 speed chain, and thus it's master link options, are narrower than a 7 speed chain. The sides of an incorrect master link will be prevented from getting close enough together to lock due to the wider chain. Using another brand's master link with a Shimano chain is common place and works just fine.
Here is a quote from Shimano's historical literature about the STX group: "Although STX was positioned as a middle-grade components group, it had the latest features, including RAPIDFIRE PLUS levers, DUAL SIS, and M-SYSTEM. What Shimano especially focused on were Advanced Light Action, Optical Gear Display, and HyperDrive-C, which were features that truly symbolized the RIDE LIGHT concept." From, 100 Products History by Shimano
As you can see, STX utilizes the technology that the CN-HG40 touts in it's marketing. Another 7 speed Shimano chain, CN-HG71, is also compatible, and would be an upgrade over the CN-HG40 due to enhanced finish and wear characteristics. Functionality would be essentially the same.
That said, you needn't be restricted to a Shimano chain at all. KMC, SRAM, Wipperman and others make 7 speed chains that will be compatible with the otherwise Shimano drivetrain. This, too, is a common practice (even bike manufacturers will utilize a different brands chain--usually KMC--despite featuring Shimano components) and there is little lost in the functionality of such a mix. As noted above, non-Shimano brand chains will come with their propietary master link.